StructureâProperty Relations in Fluorophosphate Glasses: An

Subscriber access provided by GAZI UNIV

Article

Structure-Property Relations in Fluoride Phosphate Glasses: An Integrated Spectroscopic Strategy Marcos de Oliveira, Jr., Tassia S. Goncalves, Cynthia Ferrari, Cláudio José Magon, Paulo Sergio Pizani, Andrea Simone Stucchi de Camargo, and Hellmut Eckert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11405 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

1

Structure-Property Relations in Fluorophosphate Glasses: An Integrated Spectroscopic Strategy

Marcos de Oliveira Jr,1 Tássia S. Gonçalves,1 Cynthia Ferrari,1 Claudio José Magon,1 Paulo S. Pizani,2 Andrea S. S. de Camargo1*, Hellmut Eckert1,3* 1

Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970, São

Carlos, SP, Brasil. 2

Universidade Federal de São Carlos, Departamento de Física, CP 676, 13565-905, São

Carlos, SP, Brasil. 3

Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, D-48149

Münster, Germany.

ABSTRACT A detailed structural investigation of a series of fluoride phosphate glasses with nominal compositions 25BaF2-25SrF2-(30-x)Al(PO3)3-xAlF3-(20-z)ScF3:zREF3 with x = 25, 20, and 15, RE = Yb and Eu and 0 ≤ z ≤ 1.0, and of a Sc-free set of glasses with compositions w[80 (Ba/Sr)F2-20AlF3]-(1-w)[80Ba(PO3)2-20Al(PO3)3], (w = 25, 50, 75), doped with 0.2 mole% Yb3+ or Eu3+, has been conducted. As indicated by Raman scattering and solid state NMR, the network structure is dominated by aluminum-oxygen-phosphorus linkages, which can be quantified by means of

27

Al/31P NMR double resonance techniques. The

ligand environment of the rare-earth ions is studied by (1)

45

Sc NMR of the diamagnetic

mimic Sc3+, (2) pulsed X-band EPR spectroscopy of Yb3+ spin probes, and (3) excitation and emission spectroscopy of Eu3+ dopants. The rare-earth ions are found in a mixed environment of fluoride and phosphate ions, which changes systematically as a function of glass composition. In the Sc-containing glasses the quantitative makeup of this ligand environment has been determined by

45

Sc{19F} and

45

Sc{31P} rotational echo double

resonance (REDOR). Comparison of the P- to F-ligand ratio with the batch composition

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2 indicates that the Sc3+ ions show a clear preference for phosphate over fluoride ion ligation. These REDOR results were correlated with Yb3+ EPR data, the intensity ratio of Eu3+ transitions 5D0->7F2 to 5D0->7F1, and the lifetime values of the Eu3+ emitting level 5D0. As a result, it was possible to obtain a global interpretation in terms of the associated quantitative ligand distribution (fluoride versus phosphate) in the first coordination sphere of the rare earth ions. The calibration of EPR and luminescence spectra on the basis of such solid state NMR data defines a new spectroscopic strategy for characterizing the rareearth local environments in promising laser glasses.

INTRODUCTION Rare-earth (RE) ion-doped transparent glasses and glass-ceramics are well established materials for the design of new-generation lasers and other photonic devices.1–13 Efficient dispersal of the luminescent species can be accomplished in a variety of glass matrices14–16 each one of which presents characteristic advantages and disadvantages from the application standpoint.17 Fluoride phosphate glasses play an important role in this effort. Numerous promising RE-doped compositions have been developed, characterized with regard to their photophysical properties, and functionally tested.18–25 The goal is to design a framework structure dominated by bridging oxygen links between the network formers, resulting in high mechanical stability, while, at the same time, creating a fluoridedominated low-phonon energy local environment for the luminescent ions, which favors high fluorescence quantum efficiencies and long excited-state lifetime values. To examine the validity and feasibility of this design concept, detailed information is required on the local environments of the luminescent ions. Owing to its element-selectivity and inherently quantitative character, and because the lack of long range order does not interfere with the spectroscopic information obtained, solid state NMR has developed into an extremely powerful structural characterization method for glasses.26 While the method has given invaluable information on short and medium range order of the glassy framework, the experimental characterization of the local environment of fluorescent RE ions in these glasses is complicated by the 4fn-paramagnetism of these ions, which broadens their NMR


Page 2 of 50

Page 3 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 signals beyond detectability. To overcome these problems, we have developed a threepronged strategy, involving (1) NMR studies on diamagnetic trivalent ions functioning as mimics for the luminescent species, (2) pulsed EPR studies of the paramagnetic RE ions themselves, probing direct ligation and/or spatial proximity to nuclear spins in their vicinity, and (3) the measurement of optical/photophysical characteristics. Previously, we applied this strategy to a set of new fluoroaluminophosphate glasses, with composition 25BaF2-25SrF2-(30-x)Al(PO3)3-xAlF3-(20-z)YF3-zRE3+, For each composition x two RE doped glasses were synthesized, with RE3+ = Eu3+ or Yb3+ and z = 0.2 or 5.0 mol%.27 We studied the framework by a variety of NMR techniques and characterized RE local structural using pulsed EPR methods. The results suggested that the desired chemical ordering effect may indeed be occurring in glasses with high fluoride contents. While the network structure is dominated by aluminum-oxygen-phosphorus linkages, the fluoride ions are found in mixed Al/Y/Ba/Sr environments accommodating the luminescent dopant species as well. Using Yb3+ spin probes (S = 1/2), the echo-detected EPR lineshapes show a systematic dependence on the fluoride-to-phosphate ratio of the glasses, reflecting systematic changes in the local RE ion coordination. In addition, the formation of Yb3+-F bonds is detected by hyperfine sublevel correlation (HYSCORE) spectra, which reveal strong hyperfine coupling of the Yb atoms with

19

F nuclei. Finally, photoluminescence

spectra of Eu3+-doped samples indicate that the 5D0->7F2/5D0->7F1 emission intensity ratio, the normalized phonon sideband intensities in the excitation spectra, as well as Eu3+ 5D0 excited state lifetimes, are systematically dependent on the fluoride content in the glasses. Altogether, the results suggest that the RE ions are found in a mixed fluoride/phosphate environment, which can be systematically controlled by the glass composition.27 The goal of the present study is to advance these previous qualitative insights to a more quantitative level. To this end, one may utilize the favorable spectroscopic properties of the highly abundant nuclear isotopes

45

Sc (spin – 7/2) or

89

Y (spin-1/2) as diamagnetic

mimics of the luminescent species. With an ionic radius of 83 pm, the Sc3+ ion may serve as a suitable model for small luminophores such as Yb3+ or Tm3+, (ionic radii of 86 and 94 pm, respectively), while the Y3+ ion (ionic radius 106 pm) could serve as a suitable surrogate for larger RE ions such as Ce3+, Pr3+ or Nd3+ (107, 106 and 104 pm). Between these two isotopes, the

45

Sc nuclide is better suited, owing to its larger magnetic moment



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 50

4 and more favorable relaxation characteristics. We have thus chosen to prepare a set of Sccontaining glasses investigating the local resolution

45

Sc MAS-NMR, and by

45

45

Sc coordination environments by high-

Sc/19F and

45

Sc/31P NMR double resonance

techniques. A complete network structure characterization is also performed for this system and for a Sc-free system, using 19

31

P,

27

Al and

19

F MAS NMR and

31

P/27Al,

31

P/19F and

F/27Al double resonance experiments. The results are correlated with EPR results obtained

on Yb3+-doped glasses and with photophysical measurements on Eu3+ doped samples, containing either, scandium, yttrium, or no additional RE component, with different phosphate-to-fluoride ratios. The results of the present study consolidate a new integrated spectroscopic approach for the structural characterization of promising fluoride phosphate laser glasses.

EXPERIMENTAL SECTION Sample Preparation and Characterization. Glasses with the nominal compositions summarized in Table 1 were prepared by the conventional melt quenching technique in Pt crucibles. For EPR characterization, a set of samples doped with 0.2 mol % Yb3+ was prepared and for optical characterization, a set of samples doped with 0.2 mol % Eu3+ was prepared. For doped samples A to C the YbF3 or EuF3 dopants were added at the expense of the ScF3 component. For samples D to H the RE fluoride dopants were added in excess.

Homogeneous mixtures of reagent grade precursors were melted at 1100 ⁰C for 5 min, quenched between copper plates and heat treated for 8 h at 400 °C. Mass losses due to the evaporation of volatile fluorides were monitored regularly and could be minimized to 1-2 wt. % of the batch, under these preparation conditions. Despite these minimal mass losses

previous studies had shown some unavoidable F/O exchange via the melting atmosphere.27 Fluoride quantification by

19

F NMR spectroscopy conducted on representative samples

showed that this process could be minimized, maintaining more than 80% of the batched fluoride content in the final glasses. As the scientific conclusions from this work are not dependent on the exact knowledge of the fluorine contents, the analysis was limited to representative samples, following the procedure outlined in reference 27. Differential scanning calorimetry was done on 20-50 mg samples from 25 ºC to 600 ºC on a TA


Page 5 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 Instruments DSC-2910 instrument, using a heating rate of 10 °C/min. Glass transition temperatures are reported as onset temperatures of the apparent heat capacity changes and are shown in Table 1. The results illustrate that Tg decreases systematically with increasing fluoride-to-phosphate ratio. Raman scattering measurements were performed at room temperature using a Jobin-Yvon T64000 triple monochromator with a CCD detector. The 514.5 nm line from an argon ion laser was focused on the samples by an optical microscope using a long work distance 50X objective. UV-VIS-NIR absorption spectra of optically polished samples were measured in a Hitachi spectrophotometer model U2800. Infrared and visible excitation, emission and excited state lifetime measurements on Yb3+-doped and Eu3+-doped samples were performed in a Horiba Jobin Yvon Fluorolog spectrofluorimeter model FL3-221 under similar conditions, as previously reported.25

Table 1. Nominal Compositions and Glass Transition Temperatures (Tg) of the Investigated Glasses.

AlF3

Al(PO3)3

BaF2

SrF2

Ba(PO3)2

ScF3

(mol%)

(mol%)

(mol%)

(mol%)

(mol%)

(mol%)

A

25

5

25

25

-

20

0.064

441 ± 1

B

20

10

25

25

-

20

0.136

484 ± 1

C

15

15

25

25

-

20

0.220

505 ± 1

D

15

5

30

30

20

-

0.333

454 ± 2

E

15

5

60

-

20

-

0.333

440 ± 2

F

10

10

20

20

40

-

1.00

508 ± 5

G

5

15

10

10

60

-

3.00

553 ± 5

H

5

15

20

-

60

-

3.00

539 ± 5

Sample

Solid state NMR. Solid state

31

P and

27

Al NMR and

45

Tg (ºC)

Sc/31P double resonance studies

were carried out on a Bruker Avance III triple-channel spectrometer operating at 400 MHz equipped with a 4 mm probe, which was operated at spinning rates up to 14.0 kHz.

27

Al

MAS-NMR spectra were recorded using short pulses of 1.0 µs length (corresponding to tip angles around 20º) and a relaxation delay of 10 s. 27Al triple-quantum (TQMAS) NMR data were obtained using the three-pulse z-filtering sequence,28 using hard excitation and reconversion pulses of 5.0 µs and 2.0 µs length and soft detection pulses of 10.0 µs length.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 50

6 Chemical shifts are reported relative to a 1M Al(NO3)3 aqueous solution.

31

P MAS-NMR

spectra were recorded using 90° pulses of 3.0 µs length and a relaxation delay of 150 s. In a separate set of measurements, double-quantum filtered spectra were obtained using the 1-D refocused INADEQUATE method.29 This experiment results in the selective detection of only those 31P nuclei that are involved in a P-O-P linkage (Q(1) and Q(2) units) and therefore give rise to the excitation of a double quantum coherence through indirect 31P-31P spin-spin coupling. In contrast, the signals of isolated Q(0) units are suppressed by appropriate receiver phase cycling. Experimental conditions were: spinning speed 12.0 kHz, π/2 pulse length 2.6 µs, relaxation delay 150 s. The mixing time for DQ coherence creation was 16.6 ms, corresponding to a value of the indirect coupling constant 2J(31P-31P) of 30 Hz. Chemical shifts were referenced against an external 85% H3PO4 standard.

27

Al{31P}

rotational echo double resonance (REDOR)30 measurements were conducted under the same conditions, using π recoupling pulses on the synchronized

27

31

P channel and acquisition of rotor-

Al spin echoes (the π pulse length was 6.0 µs in these experiments). Data

were acquired using the compensated method.31

45

Sc{31P} REDOR measurements were

performed using a π pulse length of 6.0 µs for 31P and 5.0 µs for 45Sc; no compensation was done in this case. The second moments M2 characterizing the dipolar interaction between the quadrupolar observation nuclei (27Al and 45Sc in the present case) and the neighboring 31

P nuclei (I = ½), were obtained by applying a parabolic fit to the REDOR data within the

range ∆S/S0 ≤ 0.2, to the expression32 ∆

=

.

(1)

These experiments were performed at MAS frequencies of 12.0 and 14.0 kHz, in order to collect a sufficient number of data points for this limited data range. The second moment values, in turn, can be calculated from internuclear distance distributions using the van Vleck expression,33 2

4 µ  1 2 2 M 2 =  0  I (I + 1)γ I γ S h 2 N −1 ∑ 6 . 15  4π  i , j rij


(2)

Page 7 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7 As previously shown, by calibrating the REDOR method on crystalline model compounds with known internuclear distances, it is possible to estimate an average number of Al-O-P and Sc-O-P linkages for the glassy materials.32 For all the nuclei detected, spin-lattice relaxation times were measured using the saturation recovery sequence. Lineshape analysis and deconvolutions were done using the DMFIT routine.34 Solid state MAS 45Sc NMR and 19

F NMR as well as

45

Sc{19F},

27

Al{19F} and

19

F{31P} REDOR experiments were

performed on an Agilent DD2 spectrometer operating at 5.7 T.

45

Sc MAS NMR and

45

Sc{19F} REDOR experiments were performed in a 4 mm probe which was operated at

spinning rates up to 14.0 kHz. 45Sc MAS-NMR spectra were recorded using short pulses of 0.5 µs length and a relaxation delay of 1 s for the glasses and 25 s for the ScF3 model compound and up to 320 scans were collected. Chemical shifts are reported relative to a 1M ScCl3 aqueous solution, using solid ScF3 as a secondary reference (δ = -52 ppm).35 45

Sc{19F} REDOR measurements were carried out using the standard Schaefer-Gullion

sequence29 (without compensation), with π recoupling pulses on the acquisition of rotor-synchronized

45

19

F channel and

Sc spin echoes. The π pulse length was 6.0 µs for

31

P

and 5.0 µs for 45Sc. Similar conditions were applied for 27Al{19F} REDOR measurements. 19

F MAS NMR spectra were recorded in 1.6 mm rotors spinning at 35.0 kHz, using a rotor-

synchronized Hahn-Echo sequence with 4 to 8 rotor cycles for the echo formation in order to remove the probe background signal. Different evolution times were tested and the values used in the experiments do not influence the relative intensities of the resolved spectral components, indicating similar spin-spin relaxation times. 90° pulses of 1.65 µs length and relaxation delays of 20 s were used. Chemical shifts are reported relative to CFCl3 using solid AlF3 as a secondary reference (−172 ppm).36

19

F{31P} REDOR

measurements were carried out in a 3.2 mm probe spinning at 20.0 kHz, using the compensation method30 with π recoupling pulses on the

31

P channel and acquisition of

rotor-synchronized 19F spin echoes. In these experiments the π pulse length was 4.0 µs for both isotopes. Homonuclear

19

F-19F dipole-dipole interactions were probed by

19

F Hahn

spin echo decay spectroscopy, utilizing π pulses of 3.3 µs length and a relaxation delay of 4 s. Following the approach previously discussed in the literature, second moments characterizing the strength of these homonuclear interactions were extracted from spin echo



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 50

8 intensities measured over the range of dipolar evolution times 16 to 60 µs, using the expression:37

=

(3)

Solid State EPR. Pulsed solid-state EPR experiments were performed on Yb-doped samples at 6 K on an E-580 BRUKER ELEXSYS X-band EPR spectrometer. Owing to very fast spin-spin relaxation no electron spin echo was observable at temperatures above 12 K. Electron spin echo envelope modulation (ESEEM) spectra were obtained at external field strengths of 550 and 700 mT using the three-pulse sequence (tp) - τ - (tp) - T - (tp) – echo,38,39 with a π/2 pulse length tp = 8 ns. The delay between the first and second pulse, τ, was set to 140 ns (different τ values were tested to examine the possible occurrence of blind spots). The time interval T was incremented in 12-ns steps starting with T = 300 ns; 300 acquisitions were accumulated for each increment with repetition times of 300 µs and up to 20 scans were added up for signal averaging. A four-step phase cycling of the first and second pulse was used for echo detection to avoid unwanted primary echoes and FID distortions.27 The resulting data were processed in the following way: the modulated echo decay was fitted to a biexponential function, which in turn is subtracted from the experimental data in order to isolate the oscillatory component. Following further apodization and zero-filling, the oscillating signal was Fourier-transformed, resulting in the ESEEM spectrum. The echo detected absorption spectra were recorded using the threepulse sequence. The integrated echo intensities were measured as a function of the magnetic field strength over the range 10 – 1000 mT. The pulse spacing between the first two pulses (τ) was set to 100 ns, and for the time between the second and the third pulse (T) a value of 10 µs was chosen in order to suppress nuclear frequency modulation effects. The Fourier-transformed spectra showed no dependence on T within the data range 8-15 µs. HYSCORE experiments were conducted at external magnetic field strengths of 450, 550, and 700 mT using the four-pulse sequence (tp) - τ - (tp) - t1 - (2tp) - t2 - (tp) – echo,38,39 with τ = 140 ns. The echo intensity was measured as a function of t1 and t2, which were incremented in steps of 16 ns from the initial values of 300 ns. Pulses of tp = 8 ns length for


Page 9 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9 the π/2 pulse and 2tp = 16 ns length for the π pulse were used to record a 160×160 matrix. Following further apodization and zero-filling (to 256×256 points), the oscillating signal was Fourier-transformed in both dimensions, resulting in the HYSCORE spectrum. A 4step phase cycle was used to eliminate unwanted echoes. Solid state HYSCORE data were simulated by the function “saffron” of the software package EasySpin® implemented in MATLAB (MathWorks, Inc).40 The simulations consider the static spin Hamiltonian for electron-nucleus spin pairs in the solid state, given by

ℋ = ∙ ∙

+ ∑& $%& % ∙ '( + ∑( ∙ ) ∙ '( ,

(4)

where the summations extend over the nuclear species considered in the simulation, the first two terms are the electronic and nuclear Zeeman interactions, respectively, where is

a symmetric tensor that can be represented by six independent components, for example, its

principal values, $** , $++ and $,, , and three Euler angles describing the orientation of its principal axes relative to a molecular coordinate system. In the case of glassy systems we

expect a distribution of Euler angles and principal values for g. Owing to lacking orientational information (which would only be available from single-crystal studies) in this work a simplified approach was taken to the simulations: all angles were considered zero (coincident g and molecular frame orientations), and strains were used in the g-parameter to represent the disorder found in glasses. The third term represents the electron-nucleus hyperfine coupling interaction. In this work axial symmetry was considered for the hyperfine tensors A, which can be described by the principal values A⊥ and A∥, with

/012 = 2/4 + /∥ ⁄3. The A- and g- principal axis systems were assumed to be coincident, due to lack of theoretical and/or experimental evidence of their relative orientation.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10

RESULTS, DATA ANALYSIS AND INTERPRETATION Raman Spectroscopy. Figure 1 shows the Raman spectra of the set of glasses. For the Sccontaining glasses A-C the relevant features include strong bands near 1085 cm-1 and 1020 cm-1, which are assigned to the symmetric P-O stretching modes of the PO3 and PO4 units, belonging to Q(1) (pyrophosphate) and Q(0) orthophosphate tetrahedra, respectively. In addition, a broad shoulder appears near 1130 cm-1 suggesting the presence of Q(2) units as well. The spectra show no evidence for P-F linkages which would be expected in the range 800-900 cm-1.41 The band observed at 750 cm-1 can be attributed to the P-O-P stretching mode involving the bridging oxygen species of Q(1) and (to a minor extent) some Q(2) species. Altogether for the glasses within series A-C the Raman peak positions show little compositional dependence (except for some minor intensity variations), indicating that the framework structure of these glasses remains approximately constant regardless of the phosphate-to fluoride ratio. For the scandium-free glasses E-H the Raman spectra indicate a network structure that tends to be more polymerized, particularly for sample H having the highest P content. In this glass the dominant feature near 1150 cm-1 indicates Q(2) metaphosphate-type units; in addition, smaller amounts of Q(1) and Q(0) units are present as well. In this sample the center of gravity of the P-O-P stretching band is shifted towards lower wavenumbers (~720 cm-1), as expected for glasses dominantly constructed from metaphosphate frameworks. As the fluoride component represented by (80BaF2 - 20AlF3) increases from w = 25 mole % to w = 75 mole %, the spectra of samples E and F show more resemblance with those of the fluoride-richer glasses A-C. The Q(2) units are largely depleted and the Q(1) and Q(0) units increase in concentration.


Page 10 of 50

Page 11 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11

Figure 1. Raman spectra of the investigated glasses. The letters refer to the compositions shown in Table 1. 31

P MAS NMR. Figure 2a shows results from single pulse 31P MAS NMR experiments for

the set of samples investigated. In agreement with data reported before on YF3-containing glasses with analogous compositions,25,27 the spectra of samples A-C exhibit a broad asymmetric lineshape, indicating multiple contributions. Deconvolution constraints were developed using DQ filtered spectra (Figure 2b) obtained with the refocused INADEQUATE technique. For the P-rich samples F and H the single pulse and INADEQUATE spectra are quite similar, indicating the absence of Q(0) species. In all the other spectra in Figure 2b two distinct components can be observed, which can be assigned to Q(1) and Q(2) units, respectively (see Table 2). The chemical shift variations arise because these species are involved in variable numbers of P-O-Al or P-O-Sc linkages. In addition, a third component can be observed in the standard MAS-NMR spectra (Figure 2a) at the high-frequency end, which is suppressed by the DQ filter. This resonance must then be assigned to Q(0) species. Again the chemical shift variations are due to the existence of variable numbers of P-O-Al and P-O-Sc linkages. Based on the final fit of the single-pulse spectra the intensity ratio for the three components (Q(0), Q(1) and Q(2)) are summarized in Table 2. For samples A-C few Q(2) species are present, as the reaction with AlF3, Al(PO3)3 and ScF3 is bound to create P-O-Al and P-O-Sc- at the expense of P-O-P linkages. Additional Q(1) and Q(0) species may arise from depolymerization via reaction with the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

12 Ba/Sr difluorides in the melt. Part of this effect may also be related to fluoride losses caused by partial fluoride/oxide ion exchange during the melting and quenching process. The oxide ions produced in this fashion are highly basic and are known to depolymerize PO-P linkages in alkali, alkaline earth and aluminum phosphate glasses. On the other hand, for samples D-H, which feature lower Al/P ratios and do not contain Sc3+ ions, larger concentrations of Q(2) species are observed. A large variation in the chemical shift for Q(1) and Q(2) groups is observed from the set of samples A-C to the set E-H. The {27Al}31P REAPDOR results, to be discussed below suggest that this behavior is due to a difference in the number of Al species in the P coordination environment.

Figure 2. Experimental

31

P MAS NMR spectra obtained by single pulse (a) and refocused INADEQUATE (b) techniques for the glasses under study. Dashed gray curves denote the deconvolutions into individual Gaussian components. Black curves denote experimental data. Letters refer to compositions shown in Table 1.


Page 12 of 50

Page 13 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13

Table 2. 31P Isotropic Chemical Shifts and Relative Areas of the Individual Q(n) Phosphate Species, Determined from the Deconvolution of the Single Pulse 31P MAS-NMR Spectra of Figure 2, and 27Al Isotropic Chemical Shift δisoCS and Second-order Quadrupolar Effects SOQE Determined from TQMAS Experiments. For Samples G and H the Attributed Coordination Numbers for the Al Sites are Given in Parenthesis. Q(0)

Q(1)

Q(2)

δiso

Area

δiso

Area

δiso

(±1 %) 49

(±0.1 ppm) -15.6

(±1 %)

A

(±0.2 ppm) -10.4

49

(±0.4 ppm) -22.2

B

-10.8

40

-16.1

58

C

-9.7

26

-13.1

D

-

-

E

-1.6

F

-

G

H

27

-

-

Area (±1 %)

SOQE (27Al) (MHz)

δisoCS (27Al) (ppm)

2

4±1

0±1

-27.6

2

5±1

-1 ± 1

70

-24.4

4

4±1

-2 ± 1

-

-

-

-

1.9 ± 0.5

-1.5 ± 0.5

34

-6.9

50

-11.9

16

3.1 ± 0.5

-1.1 ± 0.5

-

-9.1

51

-15.3

49

2.3 ± 0.5

-4.3 ± 0.5

4±1

48 ± 1 (IV)

3±1

17 ± 1 (V)

3.6 ± 0.5

-11 ± 1 (VI)

4±1

21 ± 1 (V)

2.8 ± 0.5

-9.5 ± 0.5 (VI)

-

-

-

-9.1

-

42

-

-19.5

-

58

Al-MAS-NMR. Figure 3 shows results from 27Al MAS-NMR. For the samples G and H

the 27Al MAS-NMR spectrum reveals the presence of four-, five- and six-fold coordinated aluminum sites with resonance shifts near 40, 10 and -13 ppm respectively.42-44 The compositions of these two glasses, which are characterized by the lowest fluoride contents, are close to those of NaPO3-Al(PO3)3 glasses, in which Al species in multiple coordination states have been previously observed.44 For all the other glasses the spectra indicate the dominance or exclusive presence of six-coordinated aluminum (peak maximum near 10ppm), as expected in fluoride-rich glasses.36,45 For all the samples, the resonance position is influenced by both the isotropic chemical shift (δisocs) and the second-order quadrupolar shift. Both contributions can be separated via triple-quantum MAS-NMR (TQMAS), using established data analysis procedures.28 Table 2 details the isotropic chemical shifts and the second order quadrupolar effect constants (SOQE = CQ×(1 + η2/3)1/2) obtained from this



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

14 procedure. It is well-known that the 27Al chemical shifts are quite sensitive not only to the aluminum coordination number, but also to the nature of the ligands to which these alumina coordination polyhedra are linked. For example in a series of fluoride phosphate glasses prepared by sol-gel chemistry the

27

Al chemical shift decreases with increasing aluminum

ligation with phosphorus.45 For the two sets of samples examined in the present study the δisoCS value suggests that the coordination sphere of aluminum contains a mix of 31P and 19F species and the 27Al isotropic chemical shifts decrease with increasing phosphate content in the expected manner (see Table 2). This decrease is observed within both of the series A-C and E-H, however, the trend is much more pronounced within the latter series. Quantitative insight about the Al coordination can be obtained by 27Al{31P} REDOR results discussed in the following section.

Figure 3. 27Al MAS NMR spectra for undoped glasses. The letters refer to the compositions shown in Table 1. 27

Al{31P}-REDOR and 31P{27Al} –REAPDOR. The question to which extent fluoride and

phosphate species contribute to the first coordination sphere of Al can be addressed by 27

Al{31P}REDOR studies. Figure 4 shows the results for the two sets of samples, and for

the crystalline reference compound Al(PO3)3. Dipolar second moments were obtained from parabolic analyses of the initial decay regime31,32 (see Table 3). Considering that in


Page 14 of 50

Page 15 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15 crystalline aluminum metaphosphate the second coordination sphere of Al is made up by six P atoms at a distance of 320 pm, and assuming that in glasses the same second-nearest neighbor distance is applicable, the average number of P atoms (NP) in the Al coordination environment can be deduced from the ratio M2(glass)/M2(AlPO3)3, as shown in Table 3. In principle, an analogous 27Al{19F} REDOR experiment may be used to estimate the average number of F atoms (NF) in the Al coordination environment. However, owing to the strong 19

F-27Al dipolar coupling, the signal attenuation in the 27Al{19F} REDOR experiments was

found to be so rapid that reliable values of the second moment could not be obtained in the present study. As the large majority of the Al atoms are six-coordinated in all our glasses, we instead estimated NF from the difference: NF = 6 – NP. The NP/NF ratios determined in this manner are shown in Table 3.

Figure 4. 27Al{31P} REDOR dephasing curves for undoped glasses (capital letters refer to the compositions shown in Table 1) and for an Al(PO3)3 crystalline sample. Empty and filled symbols represent experimental data points obtained under MAS spinning speed of 14.0 kHz and 12.0 kHz, respectively. Solid curves are parabolic fits to the data, analyzed within the data range ∆S/S0 < 0.2 (for the compound Al(PO3)3 a SIMPSON simulation considering a two spin system is shown).

Table 3 shows the total numbers of P-O-Al linkages, obtained by multiplying the average number of P connected to Al (from REDOR experiments) by the Al content of the glass (30 mol % for samples A-C, 20 mol % for samples E-H). Table 3 also shows the theoretical NP/NF ratio expected for a glass consisting only of Al(PO3)3 and AlF3 in a (30-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 50

16 x):x ratio. In this case we would have a structure based on polymeric Q(2) units crosslinked by Al atoms, which are octahedrally coordinated by a mix of phosphate and fluoride ions in a (30-x):x ratio. In addition, the components SrF2, BaF2 and ScF3 may be considered as network modifiers, which may depolymerize the glass structure by breaking either P-O-P or P-O-Al linkages. Glass network depolymerization by BaF2, SrF2 or ScF3 may arise from the breakage of Al-O-P linkages, and would result in NP/NF ratios in the first coordination sphere of Al that are lower than those predicted from the Al(PO3)3/AlF3 ratios alone. To some extent, this is indeed observed in the

27

Al{31P} REDOR results

obtained on the samples A, B and C, whereas for the samples E, F and H the experimental Np/NF ratio remains bigger than that predicted by the Al(PO3)3/AlF3 ratio. Overall, Table 3 indicates, however, that in all of the glasses examined the experimental NP/NF ratio in the Al coordination environment is always significantly higher than the statistically expected NP/NF ratios (considering all species in the glass formula). This result indicates that there is no randomization of fluoride/phosphate anions in the glass structure, but that the aluminum atoms preferentially associate with phosphate anions. This result confirms that the framework is dominated by P-O-Al (and some P-O-P) linkages in all of the studied glasses. Table 3. Values of Second Moment M2(P-Al) Obtained from 27Al{31P} REDOR Experiments and Connectivity Data on Undoped Glasses. NP and 7 ⁄8 *9 are respectively the Number of P Atoms and the Ratio of P to F Atoms in the Al Coordination Environment, Obtained Experimentally from REDOR Results. 7 ⁄8 :;78 is the P/F Ratio Considering only the Fluoride Ions Connected to Aluminum at the Nominal Composition. < ⁄8 =>= is the Statistically Expected P/F Ratio in the first Al Coordination Sphere. 7?:; is the Total Number of P-O-Al Linkages Obtained from the 27Al{31P} REDOR Experiments. xh

M2(P-Al) 6

2 -2

(×10 rad s )

Np

@

A C B DEF

@

GA C GH )IAH

@

GA C GH JKLK

GA−N−)I

A

0.7 ± 0.2

0.8

0.17

0.20

0.06

27

B

1.1 ± 0.2

1.3

0.30

0.50

0.14

42

C

1.6 ± 0.2

1.9

0.50

1.00

0.22

60

E

2.0 ± 0.4

2.4

0.67

0.33

0.33

48

F

2.8 ± 0.3

3.4

1.31

1.00

1.00

68

H

5.9 ± 0.7

6

>3

3.00

3.00

120

Al(PO3)3a

5.0 ± 0.1

6

-

-

-

-


Page 17 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17 a

crystalline model compound.

Figure 5. 31P{27Al} REAPDOR dephasing curves of undoped glasses (circles). (a) Results for sample B; (b) results for samples E and H. Dashed curves represent SIMPSON simulations of the REAPDOR curves for 31P interacting on average with 0.5 to two 27Al neighbors at a distance of 327 pm. For the simulations 27Al CQ values were taken from TQMAS results.

Figure 5 shows

31

P{27Al} REAPDOR dephasing curves (circles) for some

representative samples. As previous studies on the analogous Y-based glasses of samples A-C had shown the results to be independent of the compositional x-parameter,27 sample B was chosen as a representative for the 25BaF2-25SrF2-(30-x)Al(PO3)3-xAlF3-20ScF3 system, while samples E and H were selected as representatives for the Sc-free w[(Ba/Sr)F2-20AlF3]-(1-w)[80Ba(PO3)2-20Al(PO3)3]

glasses.

The

calculated 31

correspond to SIMPSON simulations considering spin-systems involving 1.5, or 2 neighboring

curves

P and 0.5, 1.0,

27

Al nuclei (on average), respectively, assuming the P-Al average

distance observed in crystalline Al(PO3)3 (327 pm) and an Al-P-Al angle of 109.4°, which is based on the O-P-O angle in the phosphorus tetrahedron. In the case of 0.5 and 1.5 Al neighbors the simulations were done considering a second moment for the dipolar coupling which is respectively three half or one half of the value expected for a

31

P-27Al spin pair.

This would correspond to an average number of Al atoms in the P coordination environment of 1.5 and 0.5. The value of CQ used in the calculations is identical with the one measured by

27

Al TQMAS, shown in Table 2. The good agreement with the

experimental data at short evolution times (∆S/S0 ≤ 0.3) indicates that the average number of P-O-Al bonds per phosphate unit is around 1.5 for sample B and around 0.5 for samples E and H. The latter two samples, which correspond to the two extreme compositions



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

18 (fluoride richest and phosphate richest samples), do not show considerable differences in the REAPDOR dephasing curves, indicating a similar average number of Al atoms in the P coordination environment.

Figure 6. Experimental

19

F MAS NMR spectra obtained by a rotor-synchronized Hahn-echo sequence. Dashed gray curves denote the de-convolutions into individual Gaussian components. Black curves denote experimental data. Spinning sidebands are marked with asterisks. 19

F NMR, 19F{31P} REDOR and 19F{27Al} and 19F{45Sc} REAPDOR. Figure 6 shows 19F

MAS NMR results for samples A-F, No spectra are reported for the phosphate rich samples G and H, which gave very low signal-to-noise ratios indicating that F-to-O exchange via the atmosphere was severe in this case. Detailed deconvolution results obtained for these spectra are summarized in Table 4. The dominant peak observed in all samples near -125 ppm can be attributed to F species in an Al rich environment.36 The comparison with the


Page 18 of 50

Page 19 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19 chemical shifts observed for ScF3, BaF2 and SrF2 model compounds, δiso = -36 ppm,35 -33 ppm46 and -88 ppm, respectively, further suggests that the component around -36 ppm in samples A-C can be attributed to F species in a mixed Sc/Ba environment, whereas the -98 ppm peak may reflect a Sr-dominated fluoride environment. This latter attribution is further supported by the absence of this signal in the Sr-free sample E. For the Sc-free samples DF various additional peaks are observed, whose chemical shifts appear to depend systematically on the Ba/Sr ratio: based on this composition dependence we assign the peak at -13 ppm in sample E to an F species in a Ba rich environment, and the signals near -42 ppm (sample E) and -33 ppm (sample F) to fluoride ions in mixed Ba/Sr environments. Finally, all Sc-free glasses show an additional small signal component near -60 ppm, which is attributed to a P-bonded F species, as confirmed by

31

P{19F} REDOR results discussed

below. Small amounts of these species may also be present in samples A and B, where they are observed at -69 ppm with a fractional intensity of about 1%, but the high degree of overlap with the other lines make the quantification of this resonance uncertain.

Figure 7. 19F{27Al} (a,b,d and e) and 19F{45Sc} (c and f) REAPDOR results for sample B (a,c,d, and f) and sample E (b and e). Upper part shows the comparison between the S0 (black curves) and S (red curves) spectra in the REAPDOR experiment for six rotor cycles (300 µs). The difference spectrum ∆S = S0 – S is also shown (blue dashed curves). Lower



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 50

20 part shows REAPDOR dephasing curves taking from the spectral intensities in the positions indicated in the plots. Resolved spinning sidebands are marked with asterisks. The

19

F resonance attributions corresponding to F-Al or F-Sc bonding can be easily

confirmed by means of 19F{27Al} and 19F{45Sc} REAPDOR experiments. Figures 7a and b show the comparison between the S0 (spin-echo) and the S (REAPDOR) spectra from the 19

F{27Al} REAPDOR experiment for samples B and E respectively, recorded with an

evolution time of six rotor periods (300 µs). The difference spectra ∆S = S0 – S are also shown. In both sets of spectra it is clearly visible that the resonance at ~ -120 ppm is much more affected by the

27

Al adiabatic inversion pulse than the other resonances. Therefore,

the resonance at ~-120 ppm can be definitively attributed to F species bonded to Al. Figures 7d and e show the individual REAPDOR dephasing curves observed for the two main resonances, indicating a faster evolution for the resonance at -120 ppm. A certain dephasing is observed for the peaks at -35 ppm and -12 ppm (in samples B and E respectively). In part this dephasing is due to the overlapping of these resonances with the spinning sideband from the resonance at -120 ppm, which cannot be avoided. In addition, weaker

19

F-27Al

dipolar coupling due to the presence of Al as distant neighbors to F may contribute to this dephasing. An analogous analysis was performed for sample B using 19F{45Sc} REAPDOR results to confirm the attribution of F-Sc species. In this case the stronger REAPDOR effect is observed for the resonance at -35 ppm (see Figures 7c and f), confirming the attribution to

19

F species in Sc-rich environments. While a quantitative analysis of these REAPDOR

curves is not possible owing to the spinning sideband contributions, the results of Figure 7 confirm the utility of such REAPDOR experiments for making resonance assignments. Figure 8 shows 19F{31P} REDOR results for samples A-C, E and F. REDOR curves were obtained for three different spectral regions, by measuring the evolution of the individual peak intensities. Figure 8a shows the REDOR curves for the resonance corresponding to Sc/Ba rich environments (around -12 to -35 ppm), while the curves for the Al/Sr rich environment are shown in Figure 8b (around -120 ppm) and Figure 8c shows the curves for the F species bound to P (around -60 ppm). Dipolar second moments were obtained from parabolic analyses of the initial decay regime.31,32 The fitted curves are shown as dashed curves in Figure 8 and the results are shown in Table 4. For the sake of comparison the REDOR curve for the sodium monofluorophosphate (Na2PO3F) model


Page 21 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21 compound is also shown in the plots of Figure 8. In this sample there are two fluoride sites, both bound to P atoms.47 Van Vleck calculations considering the P-F distances in crystalline Na2PO3F yield the theoretical second moments for the two F sites, M2(P-F) = 10.4×108 and 9.48×108 rad2s-2. A simulation considering a P-F spin-pair with an effective second moment given by the averaged M2(P-F) value is shown in Figure 8 as a solid grey curve. Comparing the results for the glasses with the results for the model compound we observe that no P-F bound species are contributing to the resonances around -12 to -35 ppm and -120 ppm. Still, sizeable second moments are obtained for these F species, indicating spatial proximity of these fluoride ions with P atoms. These results indicate that the fluoride and the phosphate species do not occur within phase separated domains. On the other hand, the REDOR curves measured for the resonance around -60 ppm (Figure 8c) show a very strong dephasing effect, confirming the attribution to P-F bonded species (In the case of sample F, the estimation of the second moment is hampered by the strong spectral overlap of this signal component with the peak near -33 ppm). Table 4. 19F MAS NMR Spectra Deconvolution for the Studied Glasses. Second Moments (M2P-F) (±10%) from 19F{31P} REDOR Dephasing Curves for the Resolved Peaks and 19F Homonuclear Second Moments (M2F-F) from Hahn Spin-echo Decay. See Text for Peak Assignments. Al/Sr-F*

Sample

Ba/Sc-F*

P-F*

δiso (ppm)

I (± 1%)

M2(P-F) /107 rad2s-2

δiso (ppm)

I (± 1%)

M2(P-F) /107 rad2s-2

δiso (ppm)

I (± 1%)

M2(P-F) /108 rad2s-2

M2(F-F) /109 rad2s-2

-125±1 -94±2

60 5

1.3 ± 0.1

-37±1

34

1.6 ± 0.1

-69±4

1

-

1.3 ± 0.5

-124±1 -97±2

59 10

0.85 ± 0.02

-35±1

30

1.0 ± 0.1

-69±4

1

-

1.6 ± 0.5

C

-126±1 -104±2

45 26

0.42 ± 0.02

-35±1

29

0.60 ± 0.02

-

-

-

1.8 ± 0.5

D

-123±1 -100±2

64 8

-

-45±3

25

-

-63±2

3

-

-

E

-116±2 -114±2

9 59

0.87 ± 0.02

-14±1

26

1.1 ± 0.1

-60±1

5

4.9 ± 0.5

1.1 ± 0.5

A

B



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 50

22

F

-115±4

82

2.1 ± 0.1

-37±2

14

2.3 ± 0.1

4

10.0 ± 0.5

0.4 ± 0.1

F{31P} REDOR dephasing curves for undoped glasses (capital letters refer to the compositions shown in Table 1) and for a crystalline Na2PO3F sample. Circles represent experimental points. Parts (a), (b) and (c) show the REDOR curves for the Sc/Ba bonded species (peak near -12 to -35 ppm), for the Al/Sr bonded F species (peak near -120 ppm) and for the Pbonded F species (peak near -60 ppm). Solid curve is a two-spin simulation for Na2PO3F based on the crystallographic P-F distance. Dashed curves are parabolic fits to the data, analyzed within the data range ∆S/S0 ≤ 0.2. Solid curves are SIMPSON simulations considering two-spin-systems.

Figure 8.

19

-64±2


Page 23 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23

Complementary insight into the spatial distribution of the fluoride ions was obtained by measuring the

19

F-19F homonuclear magnetic dipolar couplings via static spin echo decay

spectroscopy. The results of these studies are summarized in Figure 9a and Table 4. Figure 9b shows the M2F-F values plotted as a function of the molar fraction [F] in the samples. These values increase almost linearly for increasing [F], consistent with a statistical fluoride ion distribution.

Figure 9. (a) Static 19F Hahn spin-echo decay curves for undoped glasses (capital letters refer to the compositions shown in Table 1). Circles represent experimental data. Dashed curves are Gaussian fits to the initial decay (eq. (3)). (b) Second moments obtained from the Gaussian fits in (a) as a function of the fluorine concentration in the sample. The dashed line in (b) is a linear leastsquares fit to the data points.

Sc-MAS NMR. Figure 10 shows results from single pulse 45Sc MAS NMR experiments

45

for the samples A-C together with the spectrum of the ScF3 crystalline model compound (red curve). The blue dashed line in Figure 10 indicates the maximum peak position observed for crystalline Sc(PO3)3.48 The center of gravity of the 45Sc resonance is affected both by the isotropic chemical shift and by the second-order quadrupolar interaction effect. The positions observed in the spectra of Figure 10 are similar to those obtained by Mohr et al.49 for a sodium aluminophosphate glass system. In that work, TQMAS experiments indicate

45

Sc isotropic chemical shifts around 12-16 ppm, which corresponds to six-

coordinated Sc species. Considering the similarity between our spectra and those of Mohr



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 50

24 et al., we conclude that the Sc species in our glasses are also six-coordinated. The gradual high-frequency shift in the peak positions with increasing phosphate concentration (going from A to C) may indicate that the Sc3+ ions are in a mixed phosphate/fluoride environment in which the P/F ratio increases with increasing P content. However, it is clear from the data on the model compounds that the chemical shift difference between six-coordinated Sc3+in pure fluoride and in pure oxide environments is rather small. As the peak positions measured in the glasses are also likely to be affected by second-order quadrupolar shifts, we conclude that the 45Sc NMR peak positions are not going to be useful for an assessment of the number of fluoride and oxide ligands for Sc in the mixed coordination environments present in the glasses. As discussed in the following section, the fluoride to phosphate ratio in the first coordination sphere of the Sc3+ ions can be determined more quantitatively by 45

Sc{31P}- and 45Sc{19F}-REDOR experiments.

Table 5. Values of Second Moment M2(P-Sc) and M2(F-Sc) Obtained from 45Sc{31P} and Sc{19F} REDOR Experiments and Connectivity Data on Undoped Glasses A-C. Second Moment Values for the Crystalline Compounds ScPO4, Sc(PO3)3 and ScF3 are also Shown. NP and NF are the Numbers of Phosphate and Fluoride Species in the Sc Coordination Environment Derived respectively from the M2P-Sc and M2F-Sc Values, Considering that Sc is Six-coordinated (NP + NF = 6). (NP/NF)PSc are the Phosphate/Fluoride Ratios from 45Sc{31P) REDOR Results alone. (NP/NF)FSc are the Phosphate/Fluoride Ratios from 45Sc{19F) REDOR Results alone. (NP/NF) are the Phosphate/Fluoride Ratios Obtained by Direct Calculation using the Values Obtained in both Experiments. 45

Sample

M2(PSc) 6

2 -2

(×10 rad s )

Np

NF

< O P 8 7Q

M2(FSc) (×108 rad2s-2)

O

< P 8 8Q

< O P 8

A

1.4 ± 0.1

4.0 ± 0.5

1.8 ± 0.1

4.2 ± 0.5

0.43

0.43

0.43

B

2.1 ± 0.2

2.5 ± 0.5

2.7 ± 0.2

2.6 ± 0.5

0.82

1.31

1.03

C

3.1 ± 0.2

2.1 ± 0.5

4.1 ± 0.2

2.2 ± 0.5

2.16

1.73

1.86

ScPO4

6.6 ± 0.1

-

-

-

-

-

Sc(PO3)3a

4.66

-

-

-

-

-

-

5.76

-

-

-

-

ScF3 a b

b

van Vleck calculation based on crystallographic data from Ref. 50 van Vleck calculation based on crystallographic data from Ref. 52.


Page 25 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 Sc{31P}- and 45Sc{19F}-REDOR. Figure 11 shows 45Sc{31P} REDOR results for samples

45

A-C. M2 values extracted from these REDOR curves were calibrated based on parallel measurements of the model compound ScPO4 for which all the internuclear Sc….P distances are well-known from the crystal structure. In the present work the calibration factor was found to be 1.8, which was then used to correct the dipolar second moments obtained for the glasses from the parabolic analyses of the initial decay regime (dashed curves in Figure 11). Considering that in crystalline Sc(PO3)3 and ScF3 the Sc3+ ions are six-coordinated,50-52 and assuming, based on the

45

Sc MAS spectra, that in glasses the same coordination

number is applicable, the average number of P atoms (NP) in the Sc coordination environment can be deduced from the ratio M2(glass)/M2(Sc(PO3)3), as shown in Table 5.

Figure 10 – 45Sc MAS NMR spectra for samples A-C (black curves) and for crystalline ScF3 (red curve). The blue dashed line indicates the central position expected for crystalline Sc(PO3)3, obtained from Ref.48 Spinning sidebands are marked with asterisks.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

26

Figure 11. 45Sc{31P} REDOR dephasing curves of undoped 25BaF2-25SrF2-(30-x)Al(PO3)3-xAlF320ScF3 glasses and of crystalline ScPO4. Dashed curves are parabolic fits to the data, analyzed within the data range ∆S/S0 ≤ 0.2.

The number of F atoms around Sc can also be measured directly by 45Sc{19F} REDOR experiments. These data are shown in Figure 12a. However, for all the samples studied, the heteronuclear dipolar coupling is so strong that no data points within the range ∆S/S0 ≤ 0.2 are available for the parabolic fitting procedure. Therefore, we used the constant-time (CT) REDOR method,53 in which the dipolar evolution time (i.e. the number of rotor cycles) is kept at a small constant value and the position of the dephasing π-pulses is stepped through the rotation period. M2 values were determined by computer simulations, which also showed that for short evolution times applied here the full information about the detailed spin geometry of these multiple spin system is not needed. In other words, simplified twospin-system simulations suffice for the determination of second moments. The CT-REDOR curves for samples A-C are shown in Figure 12b. A two-spin-system simulation considering the second moment for the crystalline ScF3 is shown as well in this Figure, and a good agreement with the experimental data is observed for this compound, producing a calibration factor of 1.1. The simulations that best represent the experimental CT-REDOR curves for the set of glasses are also shown in Figure 12b, and the second moments considered in these simulations are summarized in Table 5. Comparing the second moments for the glasses with those expected for crystalline ScF3, the number of F atoms in the coordination sphere of Sc can be estimated. Again, based upon the assumption of six-


Page 26 of 50

Page 27 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27 coordinate Sc atoms, we can use this result to estimate the phosphate/fluoride ratio in the first coordination sphere of the scandium ions. As summarized in Table 5, the phosphate/fluoride ratios deduced from these two independent REDOR experiments are in reasonable agreement with each other. They also agree with a phosphate/fluoride ratio based on the direct quotient NP/NF given in Table 5. Note that in glasses A-C these ratios are consistently higher (by the same factor of about 7.5 ± 1.0) than the P/F batch ratios. This indicates that the scandium atoms – like aluminum – tend to prefer phosphate over fluoride ligation. Based on these results, and assuming that Sc3+ ions are valid diamagnetic RE mimics, we conclude that dominant fluoride ligation for the RE ions can only be accomplished in glasses with very low P/F batch ratios, a situation that is most closely accomplished in glasses A and B in the present study.

Figure 12. {19F}45Sc REDOR (a) and CT-REDOR (b) dephasing curves of undoped glasses A-C and for an ScF3 crystalline sample. Full and empty symbols represent data collected respectively at spinning rates of 14 kHz and 12 kHz. The solid curves are SIMPSON two-spin simulations considering the second moment (M2) calculated for the ScF3 crystal (red curve, calibration factor 1.1) and best-fit M2 values for the glasses. Dashed curves were drawn in (a) as guide for the eyes.

Assuming that Sc is six-coordinated, the value NP obtained from {31P}45Sc REDOR and the value NF obtained from {19F}45Sc REDOR should add up to a total value of six. Within the experimental errors involved, this can be considered to be the case at least for samples A and C, whereas for sample B the error seems to be a little bit higher (see Table 5). In any event, the dipolar measurements show that the Sc coordination is dominated by phosphate units for sample C, and by fluoride units for sample A, whereas in sample B the numbers of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

28 fluoride and phosphate ligands seem comparable. As discussed in the introductory section, the Sc3+ ion may be considered chemically equivalent with the smaller RE3+ ions, such as Yb3+, i.e., a mimic for these sites that can be investigated via NMR. The results obtained here are also in close agreement with the conclusions drawn from EPR results in Yb3+doped samples and from optical spectra in Eu3+ -doped samples as discussed below. Photophysical Properties of Eu3+-doped samples. To develop an understanding of the emissive properties of the rare-earth ions in these glass matrices, we measured the emission and excitation spectra, as well as the excited state lifetimes of samples doped with 0.2 mol % Eu3+. Figure 13 summarizes the emission spectra of a set of Eu-doped glasses, containing z = 0.2 mol % Eu3+. Table 6 summarizes the spectroscopic observables deduced from these spectra. As explained in Ref. [27], the intensity of the hypersensitive, electric dipole 5D0 → 7

F2 transition is highly dependent on the chemical environment of Eu3+, whereas that of the

magnetic dipole 5D0 → 7F1 transition does not depend on the local ligand field. Therefore, the ratio α = I(5D0 → 7F2)/ I(5D0 → 7F1) is usually employed as an indicator of the ligand environment of the Eu3+ site.54,55 Higher α values are expected for phosphate glasses than for fluoride glasses given the higher covalency of the RE-O bond, which results in larger admixture of excited electronic states and consequently higher transition probabilities. For Eu3+-doped fluoride phosphate glasses α values are found near unity in glasses with high F contents, whereas they are found near 2.5 to 4.0 in pure phosphate glasses.56-63 In the present system we observe α-values near 1.8, 2.2, and 2.4 for samples A, B and C, respectively and around 3.1 ± 0.2 for samples D, F and G. Thus, our data suggest a phosphate/fluoride mixed local environment for the Eu3+ ions in these glasses, being fluoride rich for sample A, intermediate in samples B and C, and phosphate richer for samples D, F and G. The same conclusion can be drawn from the excited state lifetimes monitored for the 5D0 →7F2 transition (see Table 6), which also tend to be longer for the fluoride rich glasses which provide a lower local phonon energy environment to the rare earth ions. Further evidence for a mixed fluoride/phosphate environment comes from the phonon sideband peaks associated with the excitation spectra of the monitored 5D0 → 7F2 transition (Figure 14): While the most intense excitation line lies at 464 nm, the excitation spectra also present a weak phonon sideband at 441±1 nm, which can be attributed to the


Page 28 of 50

Page 29 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


29 anti-Stokes band reflecting the vibrational wavenumber near 1100 cm-1 which contributes to the multiphonon relaxation. This band is present at the same wavenumber in the Raman spectra shown in Figure 1. It corresponds to the dominant high-frequency Raman scattering band, attributed to the Q(1) phosphate species found in these glasses. As indicated by Figure 14 the phonon sideband peaks have similar intensity for the samples A-D, and stronger intensity for the samples F and G. Comparing only samples A-C, we notice that the intensity of the phonon band is of the same order for samples B and C, and slightly less intense for sample A. This is in good agreement with the

45

Sc NMR results, where we

noticed that the mixed fluoride/phosphate Sc coordination environment in the first two samples contain more phosphate units in the former, and more fluoride in the latter. Nevertheless, for sample A, having the highest F/P ratio, both techniques strongly suggest that phosphate ligands still contribute to the Eu coordination environment to some extent (presence of phonon sideband and 45Sc{31P} REDOR effect). Although the glasses presented here are very similar to the Y-containing glasses previously published,27 a small difference can be observed in the optical parameters α and τ. Longer τ values and smaller α ratios are observed for the Y-glasses, indicating a somewhat stronger fluoride coordination of the rare-earth ions. At the same time, NMR results show that for the Sc-samples the Al atoms present a fluoride richer environment and 31

P-NMR shows a slightly different distribution of Q(n) units, showing that not only the

optical properties, but also the framework structure is slightly different in both glasses.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 50

30 Figure 13 – Emission spectra for selected glasses doped with 0.2 mol % of EuF3. In order to highlight the variations in the α ratio, the spectra are internally normalized to the peak intensity at 595 nm (5D0 → 7F1 transition). Table 6: Photophysical Data Obtained on the Fluoride Phosphate Glasses Containing 0.2 mole % Eu as a Function of Composition. The Intensity Ratio of the Emissions at 612 and 595 nm (5D0 → 7F2 and 5D0 → 7F1) is Denoted α. τ are the Average Excited State 5D0 Lifetime Values. I(441 nm) Denotes the Phonon Sideband Intensity Normalized to the Dominant Excitation Peak Intensity at 464 nm and CG Denotes the Center of Gravity of the EPR Line Determined over the Magnetic Field region 10 – 1000 mT. Data for YContaining Samples from Ref. 27 are also Included. α

τ/ms

I(441 nm) ×10-3

CG (kG)

A

1.9

4.38

5.9 ± 0.5

7.1

A’a

1.6

5.5

5.6 ± 0.5

7.3

B

2.4

3.58

7.9 ± 0.5

6.8

1.9

4.4

7.0 ± 0.5

7.1

C

2.7

3.29

7.7 ± 0.5

6.4

C’a

2.3

3.6

9.2 ± 0.5

6.7

D

3.3

2.15

11.4 ± 0.5

5.7

F

2.9

2.46

14.2 ± 0.5

4.6

G

3.2

2.33

15 ± 2

4.3

Sample

B’

a

a

Values obtained from analysis of the data for the Y-containing samples from Ref. 27 (the sample labels A’, B’ and C’ refer to analogous compositions of A, B and C, where Sc is replaced by Y).


Page 31 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31 Figure 14 – Excitation spectra recorded by monitoring the 7F2 → 5D0 transition at 612 nm for the selected glasses doped with 0.2 mol% of EuF3. The inset shows an expansion of the internally normalized phonon sideband (anti-Stokes) transition at around 441 nm. Pulsed EPR Spectroscopy of Yb-doped samples. Yb3+ is a spin S = ½ ion. The 4f configuration gives rise to a 2F7/2 term in the ground state and a 2F5/2 term in the first excited state. As the energy difference is large (10000 cm-1 for the free Yb3+ ion) the 2F5/2 term has little effect on the magnetic properties. Owing to the low dopant concentrations used (0.2 mol %), inter-atomic dipole interactions can be neglected. Figure 15 shows the echo detected field sweep (EDFS) EPR spectrum for the studied glasses. No electron spin echo is detected at zero field, consistent with the absence of rare-earth clustering.64 In agreement with the discussion given by Sen and coworkers64 we attribute the main lineshape features to the isotopologues without nuclear spin, which comprise about 70% of the natural abundance, as no specific features attributable to

171

Yb and

173

Yb nuclear hyperfine

interaction can be discerned. As observed in our previous work with Y3+-doped samples,27 Figure 15 indicates variations in the EDFS lineshapes as a function of the fluoride content of these glasses. The spectral lineshape is most likely dominated by the g-anisotropy, however, owing to the likely existence of g-tensor parameter distribution effects no unique fit to the complex lineshapes of these EDFS spectra can be accomplished. Rather, we must consider these spectra as “fingerprint” data sensitive to the mixed local fluoride/phosphate environments of the RE ions.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

32 Figure 15. Experimental echo detected absorption spectra (solid curves) for the samples A, B and C (a) and E, F and H (b). The red dashed curve is a simulation based on Yb3+ in a pure oxide environment, based on the literature.64 Figure 15 also includes a simulated EPR spectrum that represents the powder spectrum expected for isolated Yb3+ ions in GeO2 glasses, based on the data of Sen et al.64 While Yb3+ EPR data on glasses in the literature are rather scarce, previous work on phosphoaluminosilicate glasses did show significant variations of EPR lineshapes and peak positions on glass composition, suggesting that this method is sensitive to local environments.65,66 To the best of our knowledge, however, no relevant literature data are available for comparison of the present EPR spectra with those of Yb3+ ions in pure fluoride glass environments. We attribute the large changes observed in our EPR spectra to a systematic change of the oxide to fluoride ligand ratio around the Yb3+ ion, since for the phosphate richest samples (F and H) the signal is systematically displaced to lower magnetic fields, while more heterogeneous spectra, with increasingly intense contributions at high magnetic fields are observed for samples richer in fluoride. As a quantitative measure of the spectroscopic variations observed in Figure 15, we will henceforth use the average EPR spectral position (determined as the center of gravity of the EDFS spectra over the magnetic field range investigated – the values are shown in Table 6).

Figure 16. Simulated (red curves) and experimental (black curves) three-pulse ESEEM spectra for the sample B obtained using different interpulse delay times τ (indicated in the plots). The spectra were measured at magnetic field strengths of 5.5 kG (a) and 7.0 kG (b).


Page 32 of 50

Page 33 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


33 Resonances are marked with the corresponding nuclear species. Simulation parameters are described in the text. Electron Spin Echo Envelope Modulation and HYSCORE spectra. Figure 16 shows the ESEEM spectra for the glass B, at magnetic field strengths of 5.5 kG (a) and 7.0 kG (b). Four different interpulse delay times τ were used to explore the occurrence of blind spots, 100 ns, 140 ns, 180 ns and 220 ns. Clearly, the set of spectra reveal the Larmor frequencies of 45Sc (5.6 MHz and 7.1 MHz, respectively), 27Al (6.1 MHz and 7.8 MHz, respectively), 31

P (9.5 MHz and 12.0 MHz, respectively) and

19

F (22 MHz and 28 MHz, respectively),

suggesting that all four types of nuclei are interacting with the electron spins residing in the 4f orbitals of the Yb3+ ions. The indicated resonances lie in the weak-coupling limit, indicating that the hyperfine coupling constants (in Hz) are small compared to the nuclear Zeeman frequencies. Therefore, we conclude that these signals belong to nuclei in the second or higher order Yb3+ coordination sphere. Figure 16 also shows simulated spectra (red curves) for the different τ values using parameters obtained from HYSCORE studies shown below. In these simulations, small splittings have been introduced for the 45Sc, 27Al and the

31

P peaks, corresponding to anisotropic hyperfine couplings with values for the

anisotropy parameter δA of 0.02, 0.3 and 0.6 MHz, respectively, accounting for the magnetic dipole-dipole couplings between the unpaired electrons and the 45Sc, 31P and 27Al nuclei in the second coordination sphere of the Yb3+ ions. The simulations reproduce excellently the blind spot behavior observed in the experimental data.

Figure 17. Experimental three-pulse ESEEM spectra for the glass compositions studied in this work. The spectra were measured at magnetic field strengths of 5.5 kG (a) and 7.0 kG (b). Resonances are marked with the corresponding nuclear species.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 50

34

Figure 18. 31P/19F ESEEM intensity ratios (taken from Figure 16a) as a function of the nominal P/F ratio for the glasses studied in the present work and for the glasses from Ref. 27. The dashed curve represents a fit to the data. Figure 17 shows the spectra measured at 5.5 kG (part a) and 7.0 kG (part b) with τ = 100 ns for all the samples investigated. Each set of ESEEM spectra in this figure were measured under identical conditions, which allow us to compare the relative peak intensities of the observed resonances.27 As expected from the batch compositions, the 27

Al/45Sc peak amplitude ratio remains constant for samples A-C, while the

27

Al/31P

ESEEM peak ratio increases systematically with increasing Al/P ratio in the glasses. This behavior indicates that the Yb3+ ions are well dispersed in the glassy matrix, i.e. there is no evidence of phase separation or sequestration of ytterbium. Figure 18 shows the

31

P/19F

ESEEM peak intensity ratios I(31P)/I(19F) for the spectra measured at 5.5 kG (Figure 17.a) as a function of the P/F ratio of the batch. The monotonic increase in this ratio suggests that the interaction of the Yb3+ ions with the fluoride ions is successively strengthened with increasing fluoride content of the glasses. Note that the correlation also includes the previously studied glasses in the system 25BaF2-25SrF2-(30-x)Al(PO3)3-xAlF3-20YF3.27 Figure 19 shows the HYSCORE spectra obtained at 5.5 kG. All resonances were observed in the (+,+) quadrant of the spectra. The diagonal peaks correspond to the Zeeman frequencies of 27Al, 31P and 19F nuclei. In the contour plots the 45Sc cannot be resolved due


Page 35 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35 to the superposition with the 27Al signal. Such resonances near the diagonal of the spectra indicate that the electron-nuclei interaction for these species lies in the weak coupling-limit, as also observed in the ESEEM spectra discussed above. On the other hand, besides the diagonal peak at the

19

F Larmor frequency, a pair of non-diagonal resonances can be

observed in the HYSCORE spectra, which are symmetrically displaced from the diagonal position corresponding to the evidence of

19

19

F Larmor frequency. These correlation peaks give direct

F species strongly interacting with the unpaired electron in Yb3+ and are

attributed to direct Yb-F bonds. HYSCORE simulations considering an electron spin (S = ½) interacting with 27Al, 31

P and

19

F isotopes were performed in order to reproduce the experimental HYSCORE

spectra observed for the studied glasses. In all the simulations an axial g-tensor was considered, with parameters g⊥ = 1.4 and g∥ = 0.5, which is an approximation for the distribution of g values observed in the EDFS spectra. No substantial changes are observed in the HYSCORE simulations for a variation of ± 0.5 in g⊥ and ± 0.2 in g∥. Figure 20a (left) shows the simulation that best approximate the experimental spectra (spectrum for sample B shown on the right for the sake of comparison). In order to reproduce the experimental data the simulation should consider that each Yb3+ ion is interacting with two distinct types of

19

F nuclei: a strongly coupled one, giving rise to the split signal and a weakly coupled

one, with its resonance at the coupled

19

19

F nuclear Zeeman frequency. The number of weakly

F nuclei is expected to be considerably larger, due to the larger volume of the

interaction sphere considered. For the best fit this signal was multiplied by a weighting factor of 30. For the

27

Al nucleus a quadrupole coupling parameter CQ = 3.3 MHz and an

anisotropy parameter η = 0.5 was used in the simulations. This value of CQ is the average value observed in the TQMAS experiments for the set of samples and a variation of ± 1 MHz does not change substantially the simulated lineshape. The hyperfine coupling parameters used for the simulations are shown in Table 7. For 27Al and 31P isotopes only a dipole-dipole contribution is expected since they do not bond directly with Yb3+. Figures 20b and 20c show respectively simulations considering underestimated and overestimated hyperfine coupling parameters for all nuclei in order to demonstrate the effect of the hyperfine coupling parameters on the HYSCORE lineshapes. The parameters used for these simulation are also shown in Table 7.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

36

Figure 19. 2D-HYSCORE spectra recorded at a magnetic field strength of 550 mT for the glasses studied in this work. The anti-diagonal dashed lines cross the diagonal at the nuclear Zeeman frequencies for the isotopes indicated in the plots.


Page 36 of 50

Page 37 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


37

Figure 20 – 2D-HYSCORE simulated spectra (left) considering a magnetic field strength of 550 mT compared with the experimental spectrum for sample B (right). (a) Best-fit simulation; (b) Simulation with underestimated parameters; (c) Simulation with overestimated parameters. The anti-diagonal dashed lines cross the diagonal at the nuclear Zeeman frequencies for the isotopes indicated in the plots.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 50

38 Table 7 – Hyperfine Coupling Parameters Used in the Simulations Shown in Figure 20. The Errors Estimated for the Parameters Reflect the Range in which the Parameters can be Varied without Changing Substantially the Observed Lineshape. Simulation b Simulation c Simulation a Isotope A⊥ MHz A∥ MHz A⊥ MHz A∥ MHz A⊥ MHz A∥ MHz 27

Al

-1 ± 0.2

2 ± 0.5

-0.1

0.2

-2

4

-1.8 ± 0.1

3.6 ± 0.4

-0.5

1

-3

6

19 1

F

-1 ± 0.5

14 ± 1

-

-

-4

20

19 2

-0.3

0.6

-0.3

0.6

-

-

31

P

F

DISCUSSION Evolution of the Framework Structures. The results of the present study confirm that the framework structure of the present glasses is dominated by P-O-P and Al-O-P linkages. In addition, they show that the fluoride ions introduced with the network modifiers BaF2, SrF2, and ScF3 act as depolymerizing agents by breaking Al-O-P linkages. For samples D-H the breakage of Al-O-P linkages occurs to a lesser extent than for samples A-C, owing to the lower concentration of network modifiers in this case. As the NMR results indicate that only minor amounts of P-F bonds are being formed, the network modification reaction scheme can be schematically characterized by the process: Al-O-P + F- Al-F + -O-P thereby producing additional Al-F bonds and more non-bridging oxygen atoms at the phosphate sites. This mechanism can explain the successive conversion of Q(2) into Q(1) and Q(0) units detected in the NMR and Raman spectra. As evidenced by the overall results, a sizeable fraction of these fluoride species is not acting as classical network modifier species in these glasses but is present in the form of domains rich in Sr2+, Ba2+ and Sc3+, as well as some Al3+ and rare-earth ions (in case of doped samples). The F-F and F-P dipolar second moments obtained experimentally indicate that there is no phase separation between phosphate and the different types of fluoride species, and that the atomic distribution of the fluoride ions is close to random.


Page 39 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


39 Rare-Earth Ion Environments. Information about the rare-earth ions’ coordination environment was obtained by three complementary spectroscopic approaches including (i) NMR spectroscopy of the

45

Sc mimic; (ii) pulsed EPR spectroscopy of Yb3+ dopant ions

and (iii) luminescence spectroscopy of Eu3+ dopant ions. This strategy, consisting of the use of different probes with different experimental techniques, is mandatory in the present case. NMR experiments cannot be performed on paramagnetic species such as Yb3+ due to paramagnetic broadening of the lines and they are also not well suited for Eu isotopes, such as

151

Eu and

153

Eu, due to very short T2 relaxation times and the requirements of very low

temperatures (T < 10 K).67 On the other hand, the EPR technique can only be applied to paramagnetic species (such as Yb3+). Finally, owing to the hypersensitivity of Eu3+ electric dipole 5D0→7F2 transition to the ligand environment, this ion is a more suitable probe of the local structural environment than Yb3+. Altogether this set of experimental techniques can provide unique and quantitative insights into the rare-earth ion coordination environment in these glasses. Obviously, the present study is based on the assumption that the diamagnetic ion Sc3+ can be considered chemically/structurally equivalent to the paramagnetic rare-earth species Yb3+. This assumption is supported by the isostructural relationships between the corresponding oxides and phosphates.68-71 For example, scandium and ytterbium are common substituent ions in many rare-earth minerals such as gadolinite, all the sesquioxides of scandium, yttrium and ytterbium adopt the cubic bixbyite (C-type) structure,68,69 and the phosphates petrulite, ScPO4,70 and xenotime (Y,Yb)PO4,71 are isostructural. To explore the question whether Sc3+, Yb3+ and Eu3+ species can be considered chemically/structurally equivalent with respect to their coordination in the present glasses and for comparison with the previously studied Y3+-based glasses,27 we have investigated how the results obtained by the three different techniques used in the present study (e.g. NMR of

45

Sc3+, EPR of Yb3+ dopants, and luminescence of Eu3+

dopants), correlate with each other. Figure 21a shows a 3D-plot of the center of gravity CG of the Yb3+ EDFS EPR spectra as a function of the α ratio and the excited-state lifetimes τ for the present glasses and for the previously published Y-containing glasses.27 This plot shows an excellent correlation between the two optical parameters, α and τ, which is expected, since both parameters are sensitive to the Eu3+ coordination environment (e.g. the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 50

40 phosphate/fluoride ratio). An excellent correlation is also observed between CG and the optical parameters, when comparing the Sc- and Y- containing glasses while there is some scatter, when the different glass compositions D-G are included in this comparison. Figures 21b and d show 3D-plots of the P/F ligand ratio around the

45

Sc nuclei, obtained

from the REDOR NMR experiments, versus α and τ and versus α and CG, respectively. Good correlations are observed between these parameters, even though only three data points are available thus far. Figure 21c shows a 3D-plot of the normalized intensity of the phonon sideband I(441 nm) observed in the excitation spectra relative to the dominant excitation peak intensity at 464 nm (see Figure 14 and Table 6) as a function of CG and the α ratio. From this plot we can conclude that the intensity of the phonon sideband is also correlated with the other two experimental parameters, but more scattering is observed. More consistent behavior is observed when the compositional trends of I(441 nm) are considered for the three distinct series (A,B,C), (D,F,G), and (A’,B’,C’, from ref. 27), separately. Overall, the results of the present study suggest that the compositional dependence of the luminescent characteristics in rare-earth ion doped glasses may be explained on the basis of quantitative structural information about the local environments of the rare-earth ions. Furthermore the less convincing correlation between the normalized phonon sideband intensity, from excitation spectra, and the other parameters shows that this parameter is not a good one for drawing quantitative conclusions about the rare-earths’ coordination environment when glasses with different chemical components are considered. Our results also indicate that the rare-earth ions have a clear preference to coordinate with phosphate ligands than with fluoride ions. Thus, rather high F/P batch ratios are necessary to generate fluoride-dominated rare-earth local environments. In the glass system 25BaF225SrF2-(30-x)Al(PO3)3-xAlF3-20MF3 (M = Sc,Y) this situation can be achieved for x = 25 (glass A), resulting in superior luminescent properties. In contrast, in the w[80(Ba/Sr)F220AlF3]-(1-w)[80Ba(PO3)2-20Al(PO3)3] system, the F/P batch ratios that can be realized within the glass-forming region are too low to accomplish this objective. Finally, it is conceivable, that the photophysical parameters can be improved further if fluoride losses can be minimized, either by melting and quenching these glasses in oxygen-free atmospheres or by exposing the glasses to a second re-fluorination step involving re-


Page 41 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


41 melting with NH4HF2.72 These approaches will be explored during the course of future studies.

Figure 21. 3D-correlation plots for the different experimental observables obtained for the investigated glasses and those of ref.27 (a) Center of gravity of the EDFS EPR spectra (CG) versus intensity ratio α of the emissions at 612 and 595 nm (5D0 → 7F2 and 5D0 → 7F1) and the exited state 5D0 lifetime values (τ). (b) P/F ratio in the 45Sc coordination environment obtained from NMR experiments versus α and (τ). (c) Intensity of the phonon sideband I(441 nm) normalized to the dominant excitation peak intensity at 464 nm (see Figure 14) versus CG and α. (d) P/F ratio from 45Sc NMR versus α and CG. Dashed curves are guides to the eye. The labels from A-G identify the data points corresponding to the compositions investigated in this work, while labels A’, B’ and C’ refer to the Y-containing glasses27 with x = 25, 20 and 15 respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 50

42

CONCLUSIONS In conclusion, we have developed a new integrated spectroscopic strategy to advance the understanding of the local environment of the rare-earth dopants in fluoride phosphate glasses. Using scandium as a diamagnetic mimic for the luminescent rare earth species, the ligand distribution surrounding the rare-earth ions can be quantified by

45

Sc/31P and

45

Sc/19F rotational echo double resonance techniques. Based on this information, calibration

curves can be developed allowing one to deduce that particular information also from the EPR spectra and from photophysical observables. Of course, this approach makes the intrinsic assumption that the Sc3+ ion can serve as a representative ion for the paramagnetic luminophore. While this may be true for the smallest rare-earth ions such as Tm3+ and Yb3+, which have comparable ionic radii as Sc3+, the situation may be different for medium-sized (Er3+, Eu3+) or larger ions (Ce3+, Nd3+). In principle, this question can be tested by conducting analogous REDOR experiments using

89

Y or

139

La nuclei as suitable

observe-nuclei. Despite the less favorable NMR detection characteristic of these nuclei such experiments are currently under consideration in our laboratories.

ACKNOWLEDGEMENTS Authors would like to acknowledge the Brazilian funding agencies FAPESP (CEPID Project 2013/07793-6) and CNPq (Universal Projects 477053/2012-2 (HE) and 479672/2012-1 (A.S.S.C). T.S.G. is thankful to CAPES for a MSc. Fellowship. M.O.Jr acknowledges post-doctoral support by FAPESP, grant numbers 2013/23490-3 and 2015/04063-2 (BEPE). Special thanks go to Dr. Silvia Santagneli for providing access to the 400 MHz NMR laboratory at IQCAr/UNESP and to Prof. Gunnar Jeschke (ETH Zürich) for making his pulsed X-band EPR spectrometer available for this work.

AUTHOR INFORMATION


Page 43 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


43 *corresponding author: [email protected] *corresponding author: [email protected] Notes: The authors declare no competing financial interest

REFERENCES (1)

Lupei, V. Ceramic Laser Materials and the Prospect for High Power Lasers. Opt. Mater. 2009, 31, 701–706.

(2)

Kaminskii, A. A. Laser Crystals and Ceramics: Recent Advances. Laser Photon. Rev. 2007, 1, 93–177.

(3)

Ueda, K.; Bisson, J.-F.; Yagi, H.; Takaichi, K.; Shirakawa, A.; Yanagitani, T.; and A. A. Kaminskii. Scalable Ceramic Lasers. Laser Phys. 2005, 15, 927–938.

(4)

Lupei, V.; Lupei, A.; Ikesue, A. Transparent Nd and (Nd, Yb)-Doped Sc2O3 Ceramics as Potential New Laser Materials. Appl. Phys. Lett. 2005, 86, 111118.

(5)

Lupei, V.; Lupei, A.; Gheorghe, C.; Ikesue, A. Sensitized Yb3+ Emission in (Nd, Yb):Y3Al5O12 Transparent Ceramics. J. Appl. Phys. 2010, 108, 123112.

(6)

Kong, J.; Tang, D. Y.; Zhao, B.; Lu, J.; Ueda, K.; Yagi, H.; Yanagitani, T. 9.2-W Diode-End-Pumped Yb:Y2O3 Ceramic Laser. Appl. Phys. Lett. 2005, 86, 161116.

(7)

Kracht, D.; Frede, M.; Wilhelm, R.; Fallnich, C. Comparison of Crystalline and Ceramic Composite Nd:YAG for High Power Diode End-Pumping. Opt. Express 2005, 13, 6212–6216.

(8)

Lu, J.; Yagi, H.; Takaichi, K.; Uematsu, T.; Bisson, J.-F.; Feng, Y.; Shirakawa, A.; Ueda, K.-I.; Yanagitani, T.; Kaminskii, A. A. 110W Ceramic Nd3+ Y3Al5O12 Laser. Appl. Phys. B Lasers Opt. 2004, 79, 25–28.

(9)

Ma, Q.-L.; Bo, Y.; Zong, N.; Peng, Q.-J.; Cui, D.-F.; Pan, Y.-B.; Xu, Z.-Y. 108 W Nd: {YAG} Ceramic Laser with Birefringence Compensation Resonator. Opt. Commun. 2010, 283, 5183–5186.

(10)

Lupei, A.; Lupei, V. Ikesue, A. Gheorghe, C. Spectroscopic and Energy Transfer Investigation of of Yb/Nd in Y2O3 Transparent Ceramics J. Opt. Soc.Am. B-Optical



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

44 Physics 2010, 27, 1002-1010. (11)

Lan, R.; Wang, Z.; Liu, H.; Yu, H.; Guo, L.; Chen, L.; Zhuang, S.; Xu, X.; Wang, J. Passively Q-Switched Nd:YAG Ceramic Laser towards Large Pulse Energy and Short Pulse Width. Laser Phys. 2010, 20, 187–191.

(12)

Dong, J.; Ueda, K.; YagIand, H.; Kaminskii, A. A. Laser-Diode Pumped Self-QSwitched Microchip Lasers. Opt. Rev. 2008, 15, 187–191.

(13)

Lupei, V.; Lupei, A.; Georgescu, S.; Taira, T.; Sato, Y.; Ikesue, A. The Effect of Nd Concentration on the Spectroscopic and Emission Decay Properties of Highly Doped Nd:YAG Ceramics. Phys. Rev. B 2001, 64, 92102.

(14)

Heo, J.; Chung, W. J. Chalcogenide Glasses; Elsevier, 2014.

(15)

Gan, F. New Developments in Optics and Spectroscopy of Rare Earth Ion Doped Glasses. In Photonic Glasses; Gan, F.; Xu, L., Eds.; 2006; pp. 77–115.

(16)

Lucas, J.; Smektala, F.; Adam, J. L. Fluorine in Optics. J. Fluorine Chem. 2002, 114, 113–118.

(17)

de Pablos-Martín, A.; Durán, A.; Pascual, M. J. Nanocrystallisation in Oxyfluoride Systems: Mechanisms of Crystallisation and Photonic Properties. Int. Mater. Rev. 2012, 57, 165–186.

(18)

Stokowski, S.; Martin, W.; Yarema, S. Optical and Lasing Properties of Fluorophosphate Glass. J. Non-Cryst. Solids 1980, 40, 481–487.

(19)

Ehrt, D.; Carl, M.; Kittel, T.; Müller, M.; Seeber, W. High-Performance Glass for the Deep Ultraviolet Range. J. Non-Cryst. Solids 1994, 177, 405–419.

(20)

Ehrt, D. Fluoroaluminate Glasses for Lasers and Amplifiers. Curr. Opin. Solid State Mater. Sci. 2003, 7, 135–141.

(21)

Philipps, J. F.; Töpfer, T.; Ebendorff-Heidepriem, H.; Ehrt, D.; Sauerbrey, R. Spectroscopic and Lasing Properties of Er3+:Yb3+-Doped Fluoride Phosphate Glasses. Appl. Phys. B 2001, 72, 399–405.

(22)

Wang, P. F.; Peng, B.; Li, W. N.; Hou, C. Q.; She, J. B.; Guo, H. T.; Lu, M. Yb3+ Doped Fluorophosphate Laser Glasses with High Gain Coefficient and Improved Laser Property. Solid State Sci. 2012, 14, 550–553.

(23)

Kiran Kumar, K.; Jayasankar, C. K. Visible Luminescence of Sm3+:K–Ca–Li Fluorophosphate Glasses. J. Mol. Struct. 2014, 1074, 496–502.


Page 44 of 50

Page 45 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


45 (24)

Linganna, K.; Sreedhar, V. B.; Jayasankar, C. K. Luminescence Properties of Tb3+ Ions in Zinc Fluorophosphate Glasses for Green Laser Applications. Mater. Res. Bull. 2015, 67, 196–200.

(25)

Gonçalves, T. S.; Moreira Silva, R. J.; de Oliveira Jr., M.; Ferrari, C. R.; Poirier, G. Y.; Eckert, H.; de Camargo, A. S. S. Structure-Property Relations in New Fluorophosphate Glasses Singly- and Co-Doped with Er3+ and Yb3+. Mater. Chem. Phys. 2015, 157, 45–55.

(26)

Eckert, H. Short and Medium Range Order in Ion-Conducting Glasses Studied by Modern Solid State NMR Techniques. Z. Phys. Chem. 2010, 224, 1591–1654.

(27)

de Oliveira Jr, M.; Uesbeck, T.; S. Gonçalves, T.; Magon, C. J. C. J.; S. Pizani, P.; S. S. de Camargo, A.; Eckert, H. Network Structure and Rare-Earth Ion Local Environments in Fluoride Phosphate Photonic Glasses Studied by Solid State NMR and EPR Spectroscopies. J. Phys. Chem. C 2015, 119, 24574–24587.

(28)

Medek, A.; Harwood, J. S.; Frydman, L. Multiple-Quantum Magic-Angle Spinning NMR: A New Method for the Study of Quadrupolar Nuclei in Solids. J. Am. Chem. Soc. 1995, 117, 12779–12787.

29.

Guerry, P.; Smith, M. E.; Brown, S. P. 31P MAS Refocused INADEQUATE SpinEcho (REINE) NMR Spectroscopy: Revealing J Coupling and Chemical Shift TwoDimensional Correlations in Disordered Solids. J. Am. Chem. Soc. 2009, 131, 11861–11874.

(30)

Gullion, T. Rotational-Echo, Double-Resonance NMR. In Modern Magnetic Resonance; Webb, G., Ed.; Springer Netherlands, 2006; pp. 713–718.

(31)

Chan, J. C.; Eckert, H. Dipolar Coupling Information in Multispin Systems: Application of a Compensated REDOR NMR Approach to Inorganic Phosphates. Rotational Echo Double Resonance. J. Magn. Reson. 2000, 147, 170–178.

(32)

Bertmer, M.; Eckert, H. Dephasing of Spin Echoes by Multiple Heteronuclear Dipolar Interactions in Rotational Echo Double Resonance NMR Experiments. Solid State Nucl. Magn. Reson. 1999, 15, 139–152.

(33)

Van Vleck, J. H. The Dipolar Broadening of Magnetic Resonance Lines in Crystals. Phys. Rev. 1948, 74, 1168–1183.

(34)

Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calvé, S.; Alonso, B.; Durand, J.-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

46 O.; Bujoli, B.; Gan, Z.; Hoatson, G. Modelling One- and Two-Dimensional SolidState NMR Spectra. Magn. Reson. Chem. 2002, 40, 70–76. (35)

Lo, A. Y. H.; Sudarsan, V.; Sivakumar, S.; van Veggel, F.; Schurko, R. W. Multinuclear Solid-State NMR Spectroscopy of Doped Lanthanum Fluoride Nanoparticles. J. Am. Chem. Soc. 2007, 129, 4687–4700.

(36)

Chan, J. C. . C.; Eckert, H. High-Resolution 27Al-19 F Solid-State Double Resonance NMR Studies of AlF3-BaF2-CaF2 Glasses. J. Non-Cryst. Solids 2001, 284, 17–22.

(37)

Lathrop, D., Eckert, H. Structural Models for Non-Oxide Chalcogenide Glasses. Atomic Distribution and Local Order in the System Phosphorus-Selenium Studied by 31P Dipolar NMR Spectroscopy, J. Am. Chem. Soc. 1989, 111, 3536-3541.

(38)

Schweiger, A.; Jeschke, G. Principles of Pulsed Electron Spin Resonance; Oxford University Press: Oxford, 2001.

(39)

Deligiannakis, Y.; Louloudi, M.; Hadjiliadis, N. Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy as a Tool to Investigate the Coordination Environment of Metal Centers. Coord. Chem. Rev. 2000, 204, 1–112.

(40)

Stoll, S.; Schweiger, A. EasySpin, a Comprehensive Software Package for Spectral Simulation and Analysis in EPR. J. Magn. Reson. 2006, 178, 42–55.

(41)

Videau, J. J.; Portier, J.; Piriou, B Raman Spectroscopic Studies of Fluorophosphate Glasses, J. Non-Cryst. Solids 1982, 48, 385-392.

(42) Deters, H.; de Camargo, A. S. S.; Santos, C. N.; Eckert, H. Glass-to-Vitroceramic Transition in the Yttrium Aluminoborate System: Structural Studies by Solid-State NMR. J. Phys. Chem. C 2010, 114, 14618–14626. (43)

Deters, H.; de Camargo, A. S. S.; Santos, C. N.; Ferrari, C. R.; Hernandes, A. C.; Ibanez, A.; Rinke, M. T.; Eckert, H. Structural Characterization of Rare-Earth Doped Yttrium Aluminoborate Laser Glasses Using Solid State NMR. J. Phys. Chem. C 2009, 113, 16216–16225.

(44)

Zhang, L; Eckert, H. Short- and Medium-Range Order in Sodium Aluminophosphate Glasses: New Insights from High-Resolution Dipolar Solid State NMR Spectroscopy, L. Z, J. Phys. Chem. B 2006, 110, 8946-8958.

(45)

Zhang, L.; de Araujo, C. C.; Eckert, H. Structural Role of Fluoride in Aluminophosphate Sol−Gel Glasses: High-Resolution Double-Resonance NMR


Page 46 of 50

Page 47 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47 Studies J. Phys. Chem. B 2007, 111, 10402–10412. (46)

Sadoc, A.; Body, M.; Legein, C.; Biswal, M.; Fayon, F.; Rocquefelte, X.; Boucher, F. NMR Parameters in Alkali, Alkaline Earth and Rare Earth Fluorides from First Principle Calculations. Phys. Chem. Chem. Phys. 2011, 13, 18539–18550.

(47)

Durand, J.; le Cot, L.; Galigne, J. L. Etudes Structurales de Composes Oxyfluores Du P(V) II. Structure Cristalline de Na2PO3F Beta. Acta Crystallogr. B 1974, 30, 1565–1569.

(48) Mohr, D. Investigation of Scandium Coordination in Glasses and Crystalline Compounds - an NMR Study, Westfälische Wilhelms-Universität, 2010. (49)

Mohr, D.; de Camargo, A. S. S.; de Araujo, C. C.; Eckert, H. Local Environment of Scandium in Aluminophosphate Laser Glasses: Structural Studies by Solid State NMR Spectroscopy. J. Mater. Chem. 2007, 17, 3733–3738.

(50)

Smolin, Y. I.; Shepelev, Y. F.; Domanskij, A. I.; Belov, N. V. Crystal Structure of Scandium Metaphosphate Sc(PO3)3. Kristallografiya 1978, 23, 187–188.

(51)

Fedorov, P. P.; Trncova, V.; Kocherba, G. I.; Sobolev, B. P. Ionic Conductivity and Dielectric Relaxation of Scandium Fluoride. Kristallografiya 1995, 40, 716–720.

(52) Nowacki, W. Die Kristallstruktur des ScF3. Z. Kristallogr. Kristallogeom. Kristallphys. Kristallchem. 1939, 101, 273–282. (53) Echelmeyer, T.; van Wüllen, L.; Wegner, S. A New Application for an Old Concept: Constant Time (CT)-REDOR for an Accurate Determination of Second Moments in Multiple Spin Systems with Strong Heteronuclear Dipolar Couplings. Solid State Nucl. Magn. Reson. 2008, 34, 14–19. (54)

Capobianco, J. A.; Proulx, P. P.; Bettinelli, M.; Negrisolo, F. Absorption and Emission Spectroscopy of Eu3+ in Metaphosphate Glasses. Phys. Rev. B. 1990, 42, 5936–5944.

(55)

Nogami, M.; Umehara, N.; Hayakawa, T. Effect of Hydroxyl Bonds on Persistent Spectral Hole Burning in Eu3+-Doped BaO−P2O5 Glasses. Phys. Rev. B 1998, 58, 6166–6171.

(56)

Bocharova, T.; Karapetyan, G.; Mironov, A.; Tagil’tseva, N. Spectroscopic Properties of Eu3+ in Fluorophosphate Glasses. Phosphorus Res. Bull. 2002, 13, 87– 90.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 50

48 (57)

Bocharova, T. V.; Adam, J. L.; Karapetyan, G. O.; Smektala, F.; Mironov, A. M.; Tagil’tseva, N.

O.

Analysis

of Rare-Earth

Ion

Spatial

Distribution

in

Fluorophosphate Glasses in Terms of Segregation Phenomenon. J. Phys. Chem. Solids 2007, 68, 978–982. (58)

Bocharova, T. V.; Mironov, A. M.; Karapetyan, G. O. Rare-Earth-Activated Fluorophosphate Glasses: Local Environment of the Activator and Capture Volume Model. Inorg. Mater. 2005, 41, 892–899.

(59)

Vijaya, N.; Jayasankar, C. K. Structural and Spectroscopic Properties of Eu3+-Doped Zinc Fluorophosphate Glasses. J. Mol. Struct. 2013, 1036, 42–50.

(60)

Tanabe, S.; Hirao, K.; Soga, N. Local Structure of Rare-Earth Ions in Fluorophosphate Glasses by Phonon Sideband and Mössbauer Spectroscopy. J. Non. Cryst. Solids 1992, 142, 148–154.

(61)

Babu, P.; Jang, K. H.; Kim, E. S.; Vijaya, R.; Jayasankar, C. K.; Lavín, V.; Seo, H. J. Local

Field

Dependent

Fluorescence

Properties

of

Eu3+

Ions

in

a

Fluorometaphosphate Laser Glass. J. Non-Cryst. Solids 2011, 357, 2139–2147. (62)

Babu, S. S.; Babu, P.; Jayasankar, C. K.; Sievers, W.; Tröster, T.; Wortmann, G. Optical Absorption and Photoluminescence Studies of Eu3+-Doped Phosphate and Fluorophosphate Glasses. J. Lumin. 2007, 126, 109–120.

(63)

Herrmann, A.; Fibikar, S.; Ehrt, D. Time-Resolved Fluorescence Measurements on Eu3+- and Eu2+-Doped Glasses. J. Non-Cryst. Solids 2009, 355, 2093–2101.

(64)

Sen, S.; Rakhmatullin, R.; Gubaydullin, R.; Silakov, A. A Pulsed EPR Study of Clustering of Yb3+ Ions Incorporated in GeO2 Glass. J. Non-Cryst. Solids 2004, 333, 22–27.

(65) Deschamps, T.; Ollier, N.; Vezin, H.; Gonnet, C. Clusters dissolution of Yb3+ in codoped SiO2-Al2O3-P2O5 glass fiber and its relevance to photodarkening, J. Chem. Phys. 2012, 136, 014303 66.

Deschamps, T.; Vezin, H.; Gonnet, C.; Ollier, N. Evidence of AlOHC Responsible for the Radiation-Induced Darkening in Yb Doped Fiber, Opt. Express 2013, 21, 8382-8392.

(67) Comment, A.; Ansermet, J.-P.; Slichter, C. P.; Rho, H.; Snow, C. S.; Cooper, S. L. Magnetic Properties of Pure and Gd-Doped EuO Probed by NMR. Phys. Rev. B


Page 49 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


49 2005, 72, 014428. (68)

Atkinson, S. C. Crystal Structures and Phase Transitions in the Rare Earth Oxides, PhD Thesis, University of Salford, UK, 2013

(69)

Foëx, M. and Traverse, J.P. Rev. Int. Hautes. Temp. Réfract. 1966, 3, 429

(70)

Ni, X.; Hughes, J. M.; Mariano, A. N. Crystal Chemistry of the Monazite and Xenotime Structures, Am. Mineral. 1995, 89, 21-26

(71)

Bernhard, F.; Walter, F.; Ettinger, K.; Taucher, J.; Mereiter, K. Pretulite, ScPO4, A New Scandium Mineral from the Styrian and Lower Austrian Iazulite Occurrences, Austria, Am. Mineral.l 1998, 83, 625-630.

(72)

Poulain, M.; Soufiane, A.; Messaddeq, Y.; Aegerter, M. A. Fluoride Glasses: Synthesis and Properties, Braz. J. Phys. 1992, 22, 205-217 and references therein.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

50

Table of Contents Graphic


Page 50 of 50

StructureâProperty Relations in Fluorophosphate Glasses: An

Recommend Documents

StructureâProperty Relations in Fluorophosphate Glasses: An