Composition-Structure-Property Correlations in Rare-Earth-Doped

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Composition-Structure-Property Correlations in Rare-Earth-Doped Heavy Metal Oxyfluoride Glasses Carsten Doerenkamp, Eduar Carvajal, Cláudio José Magon, Walter J. G. J. Faria, Jose Pedro Donoso, Yara Galvao Gobato, Andrea Simone Stucchi de Camargo, and Hellmut Eckert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05531 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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The Journal of Physical Chemistry

Composition-Structure-Property Correlations in RareEarth-Doped Heavy Metal Oxyfluoride Glasses

Carsten Doerenkamp1, Eduar Carvajal,2 Claudio J. Magon1*, Walter J.G.J. Faria,1 J. Pedro Donoso1, Y. Galvao Gobato,2 Andrea S.S. de Camargo,1* and Hellmut Eckert1,3*

1Instituto

de Física de São Carlos, University of São Paulo, Avenida Trabalhador São-Carlense 400, 13566-590 São Carlos (SP), Brazil 2Departamento

de Física, Universidade Federal de São Carlos, Rod. Washington Luís km 235, São Carlos (SP) 13565-905, Brazil 3Institut

für Physikalische Chemie, WWU Münster, Corrensstrasse 28-30, 48149 Münster, Germany

Abstract Structure/property correlations in oxyfluoride glasses have been explored in a series of lead fluoroborate and lead fluorogermanate glasses with nominal compositions (50-x-y-z)B2O3-40PbO-y(Al2O3)-(10+x)

PbF2

and

(50-x-y)GeO2-40PbO-y(Al2O3)-

(10+x)PbF2 (x,y = 0, 10, 0 ≤ z ≤ 0.5, RE = Eu, Yb). Starting from glasses with a fixed PbF2 content of 10 mole % we explore the effects of (1) increasing PbF2 content to 20 mole % and (2) incorporating the intermediate oxide Al2O3 at the expense of GeO2 or B2O3. The emission characteristics studied on Eu-doped glasses are rationalized on the basis of structural information obtained by Raman, nuclear magnetic resonance (NMR) and pulsed electron paramagnetic resonance (EPR) spectroscopy on Yb-doped samples. In the Ge-oxyfluoride glasses, increasing PbF2 content results in enhanced excited state lifetimes of the rare earth ions, and for this system Eu3+ emission profiles, NMR and EPR results suggest an increased average number of fluoride ions in the first coordination sphere of the rare earth ions. In contrast the effect is much less apparent in the fluoroborate glasses. In both systems, Al2O3 incorporation results in pronounced changes in the fluorine speciation, indicating the formation of aluminum-fluorine bonds.

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Introduction Rare earth (RE) ion-doped glasses have received great attention due to their wide potential applications in telecommunications, lasers, solid-state three-dimensional displays, solar cells, and other photonic devices.1-5 Among these materials, transparent oxyfluoride glasses combine the advantage of low phonon energies ascribed to the fluoride matrices with a high chemical and mechanical stability provided by the oxidedominated framework structure. Therefore, they are considered to be a good choice as hosts for RE ions in optical materials.6-8 In addition, glasses containing heavy metal oxides such as Pb, Cd and Bi oxides, attract special interest owing to their high refractive indexes, low glass transition temperatures, high polarizabilities and good dispersal of RE ions.9 Of particular interest are lead fluorogermanate glasses, which combine these features with high transparency in the range from near UV to IR (0.35 – 6 μm), and which offer the prospect of preparing transparent glass ceramics, based on the crystallization of rare-earth doped cubic PbF2 by controlled thermal annealing.10-16 As substitution of expensive germanium oxide by another network former species would be highly desired for device applications, a large number of studies has been devoted to the preparation, characterization, and crystallization of lead fluoroborate glasses in the system B2O3-PbOPbF2.17-25 As it is further known that adding the intermediate oxide Al2O3 to oxyfluoride glass formulations has a favorable influence on the thermal, mechanical and chemical stability, the effect of this additive has also been investigated, albeit to a lesser extent.24,25 For further optimization of optical performance characteristics in these glasses, it will be important to establish structure-property correlations, based on suitable spectroscopic probes, such as vibrational spectroscopy, X-ray extended absorption fine structure (EXAFS) and solid state NMR. In this endeavor, investigating the detailed effects of compositional variation on the spectroscopic signatures of specific structural units and correlating these results with functional performances represents a powerful approach towards these systems. Despite the unique suitability and versatility of solid-state NMR as an elementselective, inherently quantitative method for the structural analysis of glasses,26,27 very few detailed applications to heavy metal and fluorogermanate and fluoroborate glasses have appeared to date.23,28 In the present study we investigate the influence of the network former (GeO2 or B2O3) on the optical and structural properties of photonic oxyfluoride glasses. Starting from glasses with a fixed PbF2 content of 10 mole % we will explore the 2 ACS Paragon Plus Environment

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effects of (1) increasing PbF2 content to 20 mole % and (2) incorporation of the intermediate oxide Al2O3 at the expense of the network former, upon the optical properties and the structural characteristics of these glasses. We report the preparation, and properties of a series of lead fluoroborate and lead fluorogermanate glasses with nominal compositions (50-x-y-z)B2O3-40PbO -y(Al2O3) - (10+x)PbF2 –and (50-x-y)GeO2 - 40PbO-y(Al2O3)-(10+x)PbF2 (x,y = 0, 10, 0 ≤ z ≤ 0.5, RE = Eu, Yb). The emission characteristics are studied on glasses doped with 0.5 mole% EuF3. The structural characterization of these glasses is done via Raman and multinuclear solid-state NMR spectroscopy as well as electron paramagnetic resonance, EPR, on glasses doped with 0.2 mol% YbF3. Based on the results we propose new composition-structure-property correlations in this glass system.

Experimental Sample Preparation and Characterization The starting reagents B2O3 (purity 99.98%), GeO2 (99.998%), PbO (≥99.9%), PbF2 (≥ 99%), Al2O3 (99.99%), EuF3 (99.99%), and YbF3 (99.9%), all from Sigma-Aldrich, were used without further purification. Glasses with the nominal compositions summarized in Table 1 were prepared by the conventional melt quenching technique in Pt crucibles. Samples were melted at 750-820 °C for 30-40 min in a muffle furnace and subsequently cooled rapidly by pouring the melts into a graphite mold. Owing to the low melting temperatures and limited melting times, fluoride losses are assumed to be minor or at least comparable between the different samples. Differential scanning calorimetry was done on 20-50 mg samples over a temperature range of 25 ºC to 600 ºC on a TA Instruments DSC-2910 instrument, at a heating rate of 10 °C/min. For those glasses previously reported in the literature, glass transition and crystallization onset temperatures (Table 1) are in good agreement with previous results. Raman spectra were collected on a LabRAM HR Evolution Raman spectrometer, operating at a wavelength of 532 nm (solid state laser) and using a 600 l/mm diffraction grating. The data acquisition time was 5 s with 3 accumulations, corresponding to a spectral resolution of 2 cm-1. Excitation and emission spectra were acquired in a Horiba Jobin Yvon spectrofluorimeter model Fluorolog, equipped with CW and pulsed Xe lamps as excitation sources. The signals were collected by a visible photodiode detector model PPD-850 and by an infrared Hamamatsu photomultiplier. The spectra were all corrected by the lamp profile and 3 ACS Paragon Plus Environment


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detectors' responses. The excited state lifetime values of Eu3+-doped samples were fitted from the emission decay curves in time of the most intense transition at 611 nm. Table 1: Glass compositions, glass transition and crystallization temperature onsets (Tg and Tc) of the samples under study. Oxyfluorogermanate glasses

Tg /°C

Tc(°C)

(± 2)

(± 2)

Ge1-0.5Eu: 49.5GeO2-40PbO-10PbF2-0.5EuF3

351

468

Ge2-0.5Eu: 39.5GeO2-40PbO-20PbF2-0.5EuF3

290

461

Ge3-0.5Eu:39.5GeO2-10Al2O3-40PbO-10PbF2-

299

393

B1-0.5Eu: 49.5B2O3-40PbO-10PbF2-0.5EuF3

407

-

B2-0.5Eu:39.5B2O3-40PbO-20PbF2-0.5EuF3

308

403

B3-0.5Eu:39.5B2O3-10Al2O3-40PbO-10PbF2-0.5EuF3

374

-

0.5EuF3 Oxyfluoroborate glasses

Solid State NMR. Solid state 11B, 19F, and 207Pb NMR experiments were performed on the undoped samples, at room temperature, using an Agilent DD2 600 MHz spectrometer interfaced with a 5.7 T magnet.

11B

magic-angle spinning (MAS)-NMR

spectra were measured in a 3.2 mm probe operated at a spinning speed of 25.0 kHz using short excitation pulses (20° flip angle) of 0.2 µs length and a relaxation delay of 1 s, respectively. The 19F MAS-NMR spectra were measured in a 1.6 mm probe operated at a spinning speed of 35.0 kHz using a 4-cycle rotor synchronized Hahn spin echo sequence and a relaxation delay of 100 s. 207Pb NMR spectra were obtained under static conditions, using the wideband uniform rate smooth truncation (WURST) pulse sequence,29-31 combined with the Carr-Purcell-Meiboom-Gill (CPMG) echo train acquisition scheme.32,33 WURST excitation and refocusing pulse lengths were 50.0 μs and the excitation bandwidth was set to 500 kHz. Spectra were acquired with a spikelet separation of 2 kHz, with 64 CPMG echoes, and a relaxation delay of 60 s. Fast Fourier transformation of the whole echo train resulted in spikelet patterns whose envelope represents the static lineshape. Alternatively, the envelope was obtained by summing the individual echoes of the CPMG train prior to Fourier transformation.


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27Al

MAS-NMR experiments were performed on a Bruker Avance Neo 600 MHz

spectrometer in a 2.5 mm probe operated at a spinning speed of 20.0 kHz using short excitation pulses (30° flip angle) of 0.75 µs length and a relaxation delay of 1s. All the spectra were analyzed using the DMFIT program.34 The 27Al MAS-NMR deconvolutions were carried out assuming a distribution of 27Al quadrupolar coupling constants, using the Czjzek fitting model,35 implemented within the DMFIT program.32 19F{11B}

and

27Al{19F}

19F{27Al),

dipolar recoupling experiments were done at a magnetic field

strength of 5.7 T using an Agilent DD2 spectrometer equipped with a 3.2 mm MAS-NMR probe operated at a spinning frequency of 25 kHz. The lower field strength was chosen here to minimize the spectral dispersion of the non-observed

19F

nuclei, resulting in

improved dipolar recoupling efficiency. The pulse sequence of Gullion and Schaefer was used,36 to estimate the strength of the REDOR effect after 6 rotor cycles. The lengths of the  pulses for dipolar recoupling were 5.0, 5.0, and 4.9 s for 27Al{19F}, 19F{27Al) and 19F{11B}

REDOR experiments, respectively.

Solid State EPR Pulsed solid-state EPR experiments were carried out on the Yb-doped (z = 0.2 glass samples at 5.0 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. Echo detected field sweep spectra were recorded using the three-pulse stimulated echo sequence.37,38 The integrated echo intensities were measured as a function of the magnetic field strength over a range of 10 – 1200 mT. The pulse spacing between the first two pulses () was set to 100 ns, and the time between the second and the third pulse (T) was varied between 2µs and 10μs in order to monitor (and ultimately suppress) nuclear Zeeman frequency modulation effects. Electron spin echo envelope modulation (ESEEM) spectra were obtained at external field strengths between 280 mT and 620 mT using the three-pulse sequence (tp) - τ - (tp) - T - (tp) – echo,37,38 with a π/2 pulse tp = 8 ns. The delay between first and second pulse, τ, was set to 100 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; multiple acquisitions were accumulated for each increment with repetition times of 300 µs and up to 50 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.39 The resulting data were processed in the following way: the decay of the modulated echo was fitted to a 5 ACS Paragon Plus Environment


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. HYSCORE experiments were conducted at external magnetic field strengths between 2800-6200 G using the four-pulse sequence (tp)-τ-(tp)-t1-(2tp)-t2-(tp)-echo,38,39 with τ = 100 ns. The echo intensity was measured as a function of t1 and t2, which were incremented in steps of 8 ns from the initial value of 290 ns. Pulses of tp = 16 ns length for the π/2 pulse and 2tp = 32 ns length for the π pulse were used to record a 128×128 data matrix. Following further apodization and zero-filling (to 1024×1024 points), the oscillating signal was Fourier-transformed in both dimensions, resulting in the HYSCORE spectrum. A 4-step phase cycle was used to eliminate unwanted echoes. All simulations were done using the EASY-SPIN program.40

Results and Discussion Raman spectra. Figure 1 summarizes the Raman scattering data of both glass systems. Prominent scattering peaks are observed in the region between 100 and 900 cm-1. While the intense peak at the lowest wavenumbers arise from the boson excitation, the peaks observed in the region between 400 and 500 cm-1 are generally attributed to Ge-O stretching and bending modes associated with the bridging oxygen atoms linking GeO4/2 units. In addition, a strong scattering peak near 800 cm-1 with a shoulder near 720-730 cm-1 is observed. Based on the Raman spectrum of crystalline PbGeO3 this peak is most plausibly assigned to Ge-O vibrations of four-coordinated Ge(2) units containing two non-bridging oxygen atoms.41,42 For the Al-containing sample, the Raman scattering profile in this spectral region shows a significant difference, suggesting a major degree of re-structuring of the Ge-based local environments, which may be interpreted in terms of Ge-O-Al linkages replacing Ge-O-Ge linkages. The spectra of the borate glass system look considerably more complex, showing seven features, as labeled in Figure 1b. Following the relevant literature on lead fluoroborate glasses,43-45 their assignments are 1. Boson peak, 2 and 3, Pb-O stretching modes, 4. metaborate ring, 5. B-O stretching mode of metaborate units, 6. orthoborate units, 7. pyroborate units. In glasses B2 and B3, the


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Raman band of the latter unit appears to be shifted to somewhat lower wavenumbers compared to the situation in glass B1.

Figure 1: Raman scattering spectra of (a) the fluorogermanate glasses Ge1, Ge2, and Ge3, and (b) the fluoroborate glasses B1, B2, and B3. The inset shows a vertical expansion. Peak labels denote 1. Boson peak, 2 and 3, Pb-O stretching modes, 4. metaborate ring, 5. B-O stretching mode of metaborate units, 6. orthoborate units, 7. pyroborate units. Photophysical properties Figure 2 shows the typical emission spectra of the six samples and their related optical transitions, upon excitation at 390 nm. From the luminescence decay curves in time (exc = 390 nm, em = 611 nm), the average lifetime values of excited state 5D0 were determined by single exponential fits, for all the samples. As reported in Table 2, together with the emission bands intensity ratio  and phonon frequencies νphonon, a single exponential lifetime component is expected and verified for emitting level 5D0, due to the very low dopant concentration. While the electric-dipole allowed 5D0 → 7F2 transition is highly sensitive to the ligand environment, the other emission peaks are mostly magnetic dipole allowed and much less sensitive to the Eu coordination. Thus, one customarily considers changes in the integrated peak intensity ratio



I 5 D 7 F 0

2

I 5 D 7 F 0

1



as a function of composition as a qualitative measure of changes in the local environment of the rare-earth ions. For example, results obtained on oxyfluoride glasses have previously shown that  - values decrease systematically with increasing F/O ratio of the glass composition, reflecting a decrease in the overall bonding covalence as F replaces O in the rare-earth’s first coordination sphere.28,46-48 A second characteristic photophysical parameter is the excited state lifetime, determined by pulsed excitation experiments. Figures S3 and S4 illustrate slight deviations from exponential decay behavior, which may originate from a distribution of lifetimes. Table 2 lists two alternative values: the average lifetimes AV obtained via integration of the full data, and values fit obtained by approximating the initial decay (up to 3 ms) by an exponential function. The differences between AV and fit are of similar magnitude in all of the samples, suggesting that there are no substantial variations in the width of the distribution functions of lifetimes. In the present glass system, we also observe moderate changes in  - values and lifetimes as a function of composition. For the fluorogermanate glasses our finding corresponds exactly to the trends expected: as PbF2 content increases from 10 to 20 mole%, there is a decrease in  - values and an increase in lifetime Based on these results glass Ge2 appears to have a higher degree of participation of fluoride in the Eu3+ coordination sphere than glasses Ge1 and Ge3. In contrast, for the fluoroborate glasses, an increase in the content of PbF2 or Al2O3 seems to have a relatively small effect on either  - values and lifetimes, at least over the limited compositional range studied here. (Work in reference 28 has shown, however, that longer lifetimes up to ~3.0 ms can be measured in lead fluoroborate glasses with substantially higher PbF2 contents). For the glasses of the present study, the vibrational relaxation associated with the ligation of Eu3+ to the borate anions seems to remain the dominant de-excitation mechanism. This is corroborated by a closer look at the excitation spectra discussed below.


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Figure 2: Emission spectra of all the fluorogermanate and the fluoroborate glasses, doped with 0.5 mole% EuF3 examined in the present study (exc = 393 nm). The labels belong to the transitions 0. 5D0→7F0, 1. 5D0→7F1, 2. 5D0→7F2, 3. 5D0→7F3, 4. 5D0→7F4.

Table 2. Photophysical parameters, peak intensity ratio α (see text), excited state lifetime AV calculated via 𝜏𝐴𝑉 = ∫𝐼(𝑡) ∙ 𝑡 𝑑𝑡 ∫𝐼(𝑡)𝑑𝑡 and exponential fit fit  and phonon frequency phonon of the fluorogermanate and fluoroborate glasses doped with 0.5 mole% EuF3 under study.



AV /ms fit/ms

phonon/cm-1

±

(±0.02)

(±25)

Ge1:49.5GeO2-40PbO-10PbF2

2.8

1.37

1.391 ± 0.002

771

Ge2:39.5GeO2-40PbO-20PbF2

1.9

1.77

1.817 ± 0.002

746

Ge3:39.5GeO2-10Al2O3-40PbO-10PbF2

2.8

1.48

1.441 ± 0.009

745

B1: 49.5B2O3-40PbO-10PbF2

3.0

1.72

1.687 ± 0.005

1308/1024

B2: 39.5B2O3-40PbO-20PbF2

2.6|

1.56

1.526 ± 0.003

1232/966

B3:39.5B2O3-10Al2O3-40PbO-10PbF2

2.9

1.58

1.612 ± 0.003

1279/1021

Glass composition



Figures 3a and 3b show the excitation spectra monitoring the most intense emission peak corresponding to the hypersensitive 5D0 → F2 transition at 611 nm, indicating the respective band assignments. As it is characteristic of Eu3+ doped materials, the most intense excitation bands are around 390 nm (7F0 → 5L6 transition) and 462 nm (7F0 → 5D

2)

(see energy level scheme in Figure 2). Another significant information extracted

from these spectra are the phonon sideband frequencies,49 involved in the de-excitation process of the 5D0 excited state (see vertically expanded features in Figure 3). Table 2 indicates that the positions of these sidebands differ significantly between the fluorogermanate and the fluoroborate glasses. For the fluorogermanate glasses, these sidebands are observed in the range 447.5 to 448.5 nm, corresponding to vibration wave numbers in the 750-770 cm-1 range. In the context of Figure 1a, we may conclude that vibrations affecting Ge-bound non-bridging oxygen species (Raman peak near 750-790 cm-1) respectively, contribute significantly to the de-excitation process. Note that the differences in the vibrational wavenumbers observed in the Raman spectra of glasses Ge1 and Ge3 are also reflected in the optical excitation spectra. For the fluoroborate glasses, the de-excitation relevant phonon energies are found to be significantly higher, and two contributions are observed, near 438 and 442 nm, corresponding to vibrational frequencies near 1300 and 1000 cm-1. Comparison with the Raman spectra (Figure 1) suggests that the type of borate species coordinating to Eu3+ (responsible for de-excitation of 5D0) are anionic three-coordinated borate species with non-bridging oxygen atoms (ortho- and pyroborates). Again, the phonon sideband positions indicate that the deexcitation frequency is detectably higher in glass B1 than in glasses B2 and B3, in agreement with the Raman spectra.


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Figure 3a: Excitation spectra of the fluorogermanate and fluoroborate glasses of the present study, monitoring the 5D 0→7F2 emission peak at 611 nm.


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

Intensity (CPS/A)


6x10

7

5x10

7

4x10

7

3x10

7

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B2-0.5Eu Ge2-0.5Eu

5

L6

438nm

448nm

5

D2

430

435

440

445

450

(nm)

2x10

7

5

1x10

7

5 5

D4

L7 5

GJ

D1

5

D0

5

D3

0 350

400

450

500

550

600

(nm)

Figure 3b: Excitation spectra of Eu doped glasses B2 and Ge2, monitoring the 5D 0→7F2 emission peak at 611nm, including a vertical expansion of the phonon sidebands, associated with the excitation peak near 463nm. EPR Spectra. X-band EPR spectra recorded in the echo detected field sweep mode on the YbF3 doped samples are shown in Figure 4. 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 (10,000 cm-1 for the free Yb3+ ion) the 2F5/2 term has little effect on the magnetic properties of trivalent ytterbium. Furthermore, owing to the low dopant concentrations used (0.2 mol %), inter-electronic dipole interactions can be neglected. No electron spin echo is detected at zero field, consistent with the absence of rare-earth ion clustering (contrary to what was previously observed for Yb3+ ions in pure GeO2 glass.50). No specific features attributable to

171Yb

and

173Yb

nuclear hyperfine interactions, as

visible in crystalline -PbF2 containing glass-ceramics,51 can be discerned. The EDFS line shape is most likely dominated by spin-orbit coupling and the electrostatic interaction of the rare-earth ion with the crystal field. Owing to the likely existence of interaction parameter distribution effects typical for the local variations in glassy materials, no unique fits to the complex line shapes of these spectra are possible, even though systematic line 12 ACS Paragon Plus Environment

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shape variations with glass composition are evident in the case of the fluorogermanate glasses. Figures 4-6 summarize our fitting approach: in the case of the fluoroborate glasses (Figure 4) the signals can be simulated assuming a rhombic g-tensor with large gstrain values associated with its three components. The values of gzz may be subject to a systematic error, which arises from the fact that there is still considerable signal intensity at the largest magnetic field value accessible; thus the signal had to be artificially extrapolated in a Gaussian fashion to higher magnetic fields in order to arrive at a plausible fit value for this component. The spectra of the fluorogermanate glasses (Figure 5) look more complex. Aside from a strong component C1, dominating the signal at high magnetic field strengths, there are some weaker low-field features that must be included in the fit: a sharp component near g = 2.045 (origin and assignment uncertain), and another rhombic component C2 in the low-field region. Compelling evidence for these two distinct components comes from the variable echo delay experiments done on sample Ge3 (see Figure 6), revealing different spin-spin relaxation times. Table 3 summarizes all the fitting parameters. In the fluoroborate glasses, the average g-tensor components and their associated strains are very similar for the three glasses. We assign these parameters to Y3+ sites in mixed fluoride/oxide environments with a distribution of F/O ratios. In contrast, in the fluorogermanate glasses the average g-value gav of the dominant component C1 decreases in the order Ge1 → Ge3 → Ge2, suggesting a systematic change in the local environment of the rare earth species. Systematic compositional changes in gav have been previously observed in Yb~doped fluoride phosphate glasses.47 These changes are correlated with corresponding trends in the photophysical parameter  and the excited state lifetimes  measured on analogous Eu3+ doped samples. Therefore we suggested that gav is a qualitative measure of the average O/F ratio in the first coordination sphere of the rare earth ion.47 Based on this working hypothesis we conclude that there are distinct differences between the fluoroborate and the fluorogermanate glasses of the present study. The fluoroborate (B1, B2, B3) have rather similar gav values and relatively constant excited state lifetimes, and they also show a limited compositional variation of values. All of this suggests that the average O/F ratios in the first coordination sphere of Yb3+ are relatively similar for these samples. In contrast, in the fluorogermanate glasses gav,  and av vary significantly with the glass composition, suggesting – in analogy to the situation in fluoride phosphate glasses, significant differences in the average O/F ligand ratio. Finally, based on the 13 ACS Paragon Plus Environment


values of gav, the minor components C2 identified in these glasses are assigned to a purely oxidic environment. Owing to complex electronic, structural and dynamic factors affecting the numerical values of gav,  and av we suspect that a direct comparison of these parameters between the fluoroborate and the fluorogermanate glasses is not possible; the interpretation should be limited to the compositional trend observed within each family of network former system.

B1 B2 B3 0

2500

5000

7500

10000

Magnetic field (Gauss)

Figure 4: X-band EDFS EPR spectra of the fluoroborate glasses doped with 0.2 mole% YbF3. The delay time between the second and the third pulse (T) is 2 s in all cases, the excitation band width is 100 MHz. Black trace represents the experimental data and red the simulated spectra. Microwave frequency: 9.385 GHz.


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Ge1 - 2000 ns

C1

(a) C2

C3

Ge2 - 2000 ns C1

(b) C2

C3

Ge3 - 2000 ns

C1

(c) C2 C3 0

2500

5000

7500

10000


Figure 5: X-band EDFS EPR spectra at 5 K of the fluorogermanate glasses doped with 0.2 mole% YbF3. The delay time between the second and the third pulse (T) is 2 s in all cases, the excitation band width is 100 MHz. Black trace represents the experimental data and red the simulated spectra. The three components, C1, C2, and C3 are shown in blue (see text). Intensity ratios are: (a) C2/C1 = 0.056, (b) C2/C1 = 0.039 and (c) C2/C1 = 0.26. Microwave frequency: 9.385 GHz.



C1

Ge3 - 3000 ns Ge3 - 2000 ns

(a)

Page 16 of 32

(b)

C2 C3

0

Ge3 - 10000 ns

Ge3 - 4000 ns

(c)

2500

5000

7500

(d)

10000

0


2500

5000

7500

10000


Figure 6: X-band EDFS EPR spectra at 5 K of the Ge3 fluorogermanate glass doped with 0.2 mole% YbF3, as a function of the delay time between the second and the third pulse (T). The excitation bandwidth is 100 MHz. The corresponding values of T are indicated in the figure. Black trace represents the experimental data and red the simulated spectra. The three components, C1, C2, and C3 are shown in blue (see text). Intensity ratios are: (a) C2/C1 = 0.26, (b) C2/C1 = 0.20, (c) C2/C1 = 0.16 and (d) C2/C1 = 0.034. Microwave frequency: 9.385 GHz. Table 3: Spin Hamiltonian and broadening parameters used for the numerical simulation of the EDFS spectra of fluorogermanate and fluoroborate glasses doped with 0.2 mole% YbF3. (gxx, gyy, gzz) are the principal values of the tensor g. For the germanate glasses there are two components C1 and C2. Strains in g-values are denoted as (gSxx, gSyy, gSzz) and represent the FWHM of the Gaussian distributions. The average g-value is gav = (gxx + gyy + gzz)/3. gxx gyy gzz gav gSxx gSyy (±0.05) (±0.05) (±0.05) (±0.05) (±0.05) B1 1.63 1.07 0.46* 1.05 0.56 0.55 B2 1.62 1.10 0.47* 1.06 0.55 0.67 B3 1.51 1.03 0.46* 1.00 0.59 0.57 Ge1–C1 2.24 1.42 0.62 1.43 0.23 0.86 Ge2–C1 1.80 1.19 0.53 1.17 0.23 0.83 Ge3–C1 2.05 1.16 0.62 1.28 0.55 0.41 Ge1–C2 4.08 2.75 2.14 2.99 0 0.35 Ge2–C2 4.17 2.42 2.15 2.91 0 0.82 Ge3–C2 2.52 2.34 1.44 2.10 0.36 0.68 *fit value based upon Gaussian extrapolation of the experimental data 16 ACS Paragon Plus Environment

gSzz (±0.05) 0.24* 0.26* 0.24* 0.43 0.34 0.45 0 0 0.54

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Figure S5 summarizes the electron spin echo envelope modulation (ESEEM) spectra of both glass systems, recorded under avoidance of any blind-spots. The magnetic field position for recording these spectra was chosen to correspond to the field at which the EDFS spectra have their peak maxima. Note that all the elemental constituents of these glasses with available nuclear magnetic isotopes (10B, 11B, 19F, 27Al, and 207Pb) are being detected at or close to their Zeeman frequencies. This is the result expected in the weakcoupling limit, where the hyperfine coupling constant is relatively small compared to the nuclear Zeeman frequency. Particularly intense peaks at the

19F

and

207Pb

Larmor

frequencies indicating such spatial proximity to fluoride were observed in the germanate glasses Ge1 and Ge2, but not in Ge3 glass nor in the fluoroborate glasses of the present study. These differences may indicate some segregation of Yb3+ into a PbF2 rich domain also suggested by the 19F NMR data discussed below. Yb3+-19F interactions in the weak coupling limit are detectable, however, in all of the glasses using HYSCORE experiments, see Figure S6. In the case of glass Ge1 substantial off-diagonal intensity indicative of direct Yb-F covalent bonding can be observed. Figure 7 shows an example for glass Ge1, recorded at two magnetic different field strengths, but simulated with the same set of interaction parameters. Consistent with the ESEEM spectra, the diagonals further show the expected weak interactions of the unpaired electrons with the nuclei 11B, 10B, 27Al,

and

207Pb.

All of the results obtained are consistent with a nanoscopically

homogeneous glass structure and Yb3+ ion distribution of the fluoroborate glasses B1, B2, and B3, as well as glass Ge3.



Figure 7: HYSCORE spectra of the Ge1 glass at 5 K under two experimental conditions: (a) magnetic field = 280 mT and  = 136 ns, (b) magnetic field = 510 mT and  = 104 ns. Left: experimental data. Right: simulated spectra, based on an excitation bandwidth of 100 MHz, corresponding to a pulse width of 8 ns. The spin Hamiltonian and broadening parameters used in both simulations are the same: g = (2.2062, 1.4106, 0.6244); Magnetic field strain (H-Strain) = (2873, 5929, 6737) MHz; hyperfine coupling (isotropic, axial, rhombic) constant: A(19F) = (2.5, 6.0, 0) MHz and A(207Pb) = (0.5, 0.5, 0) MHz.


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Network Structural Characterization by solid-state shows the

11B

11B

and

27Al

NMR. Figure 8

MAS NMR spectra of the fluoroborate glass series, indicating similar

results as previously observed in our earlier study of lead fluoroborate glasses.28 Threeand four-coordinated boron units can be easily resolved. The lineshape of the threecoordinate (B(3)) species is characterized by second-order quadrupolar broadening. For the four-coordinate (B(4)) species, only a relatively sharp resonance with an approximately Gaussian line shape is observed, as the environment is much more symmetric and the influence of the quadrupolar interaction on the lineshape is very weak. Table 4 summarizes the lineshape parameters. For the two glasses B1 and B2 the fractions of fourcoordinate boron, N4, are found to be identical within experimental error. Based on the PbO/B2O3 ratios R = 0.8 and 1.0, respectively, a larger fraction of four-coordinate boron is expected for the former sample. On the other hand, the larger fluoride content of glass B2 may well result in more F-bonded four-coordinate boron species, which might compensate this difference. The alumina-containing glass B3 (R = 1) shows a much lower amount of four-coordinate boron species than the other two fluoroborate glasses. Here, the successful competition of Al with boron species for the network modifier (PbO) and for bonding to fluoride ions may be responsible. The preferential attraction of network modifier by aluminum over boron in glasses containing both network former species has been previously characterized in detail in aluminoborate glasses.52



Page 20 of 32

Figure 8. 11B MAS-NMR spectra of the fluoroborate glasses, measured at 5.7 T. Individual deconvolution components simulating these spectra in terms of superimposed B(3) and B(4) resonances are shown. Table 4: Deconvolution and NMR interaction parameters (isotropic chemical shift isocs, quadrupolar coupling constant CQ and electric field gradient asymmetry parameter Q) of the 11B MAS-NMR lineshapes in the glasses under study. B(3) B(4) Sample

%

δisocs/ppm

ηQ

CQ

%

δisocs

(±1)

(±0.5)

(±0.1)

(MHz)

(±1)

/ppm

(±0.1)

(±0.5)

B1

51

21.7

0.20

2.6

49

0.8

B2

50

21.1

0.20

2.6

50

1.3

B3

65

19.7

0.20

2.6

35

0.8

Figure 9 and Table 5 summarize the results of 27Al MAS-NMR spectroscopic studies on the glasses Ge3 and B3. The distributions of the aluminum environments in the two glasses are almost identical, revealing the presence of Al(4), Al(5), and Al(6), respectively. The connectivity of these aluminum species was further investigated by 20 ACS Paragon Plus Environment

27Al{19F}

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REDOR experiments done at 5.7 T, and the results are illustrated in Figure 10. While at this lower field the resonances for Al(4) and Al(6) are still easily distinguishable, the peak from Al(5) is not resolved in the regular spectra. Nevertheless, the Al(5) species are clearly visible in the difference spectra. The large differences in dephasing caused by dipolar recoupling of the 19F nuclei clearly prove that the higher-coordinated Al(5) and Al(6) are both F-bonded, whereas the dominant Al(4) species are not. This finding is in excellent agreement with results in related Al-containing oxyfluoride glasses, such as the system NaPO3-AlF3.53 Based on these REDOR results, we assign the signals in the range of 34 to 38 ppm and 6 to 8 ppm to AlO4/2F2- units (five-coordinate aluminum, Al(5)) and to AlO4/2F23- units (six-coordinate aluminum, Al(6)), respectively. The small chemical shift differences within the groups of Al(4), Al(5), and Al(6) species in glasses Ge3 and B3 can be attributed to differences in the second coordination spheres of these units (Al-O-Ge versus Al-O-B linkages). We further note that the 27Al{19F} REDOR data suggest that there may also be some Al(5) and Al(6) units that are not F-bonded. This finding is supported by additional

27Al

MAS-NMR spectra of glasses in the system 40B2O3-

10Al2O3-(50-w)PbO-wPbF2, which also show such units for the composition w = 0 (see Figure S7). This figure further shows that the concentrations of the Al(5) and Al(6) units increase successively with increasing w, again consistent with the increase of the Al coordination number when fluoride is added to the glass melt.

Figure 9: 27Al MAS-NMR spectra of glasses Ge3 (left) and B3 (right) at 14.1 T, and their deconvolutions into three components represented by Czjzek distributions, corresponding to Al(4), Al(5) and Al(6) units.



Page 22 of 32

Figure 10: 27Al{19F} REDOR of glass Ge3 (left)and B3 (right) after 6 rotor cycles (NTr = 240 µs), measured at 5.7 T. While the resonances for Al(4) and Al(6) are easily distinguishable, the peak from Al(5) is not resolved in the regular spectra at this lower field strength. Nevertheless, the REDOR difference spectra (blue curves) clearly indicate that the Al(5) and Al(6) units are bonded to F while the Al(4) units are not. Table 5: Deconvolution and interaction parameters (isotropic chemical shift isocs and average quadrupolar coupling constant CQ of the 27Al MAS-NMR spectra at 14.1 T of 40GeO2-10Al2O3-40PbO-10PbF2 and 40B2O3-10Al2O3-40PbO-10PbF2 glasses. Al(4)

Sample

Al(5)

Al(6)

%

δisocs/ppm

CQ/MHz

%

δisocs/ppm

CQ/MHz

%

δisocs/ppm

CQ/MHz

(±2)

(±0.5)

(±0.1)

(±2)

(±0.5)

(±0.1)

(±2)

(±0.5)

(±0.1)

B3

66

63

4.9

16

34

5.3

18

6

3.8

Ge3

69

66

4.1

12

38

5.9

18

8

2.9

Figure 11 shows the 19F MAS-NMR spectra observed for both glass systems, including their deconvolutions into up to three Gaussian lineshape components, whose parameters are summarized in Table 6. The

19F

NMR spectra of the boron-free fluorogermanate

glasses show a dominant signal near -45 ppm (F1). Based on previous NMR results obtained on lead-containing oxyfluoride glasses,23,28 we can assign this resonance to fluoride species in a local environment dominated by Pb2+ ions. No significant changes 22 ACS Paragon Plus Environment

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are noticed when the PbF2 content is increased from 10 to 20 mole %. In contrast, the substitution of GeO2 by an equimolar amount of Al2O3 (glass Ge3) produces a major change in the spectra. Here, the majority of the 19F nuclei (~60%) now contribute to a broad resonance near -98 ppm (F2), accompanied by a shoulder near -138 ppm (F3). Both of the signals F2 and F3 must be assigned to Al-bound fluoride ions, probably engaging in different modes of bonding.

Figure 11 19F MAS-NMR spectra of the glasses under study. Left: fluorogermanate, right: fluoroborate glass system. The spectra of the fluoroborate glasses B1 and B2 resemble those of compositionally related samples previously studied in reference 28. Again, we attribute the -55 ppm signal to fluoride ions whose environments are dominated by Pb2+ ions (F1’), whereas the signal near -105 ppm (F2’) can be conclusively assigned to 19F nuclei in BO3/2F- units. This is confirmed by previous

19F{11B}

REDOR and

11B{19F}

heteronuclear correlation

experiments in related glasses.23,28 As expected this latter signal decreases in intensity when the B/F ratio is decreased in the sample B2 containing 20 mole % PbF2. These results indicate that the F- distribution is mainly the result of the competitive attraction of fluoride by Pb2+ and B3+, a mechanism that is not operative in the fluorogermanate glass 23 ACS Paragon Plus Environment


Page 24 of 32

system, where we find no evidence for Ge-F bond formation. If, however, B2O3 is substituted by an equimolar amount of Al2O3 (glass B3) a very similar 19F MAS-NMR spectrum is observed as in the Al2O3 substituted fluorogermanate glass Ge3. The significantly increased intensity of the F2’ signal near -105 ppm and the new species near -145 ppm (F3’) suggests again that both effects arise from the appearance of Al-bonded fluoride species.

Table 6: Isotropic chemical shift isocs and area fractions (in %) obtained for the three deconvolution components of the 19F MAS-NMR spectra of the glasses under study. sample δisocs1 (± % δisocs2 (± % δisocs3 (± % 0.5)/ppm

(±2)

0.5)/ ppm

0.5)/ ppm

(±2)

Ge1

-45.2 (F1)

100

--

--

--

--

Ge2

-45.8 (F1)

100

--

--

--

--

Ge3

-49.0 (F1)

27

-98 (F2)

60

- 138 (F3)

13

B1

-54.4 (F1’)

74

-105.4 (F2’)

26

-

--

B2

-54.8 (F1’)

89

-103.9 (F2’)

11

-

--

B3

-57.4 (F1’)

28

-104.0 (F2’)

48

-145 (F3’)

24

Thus, in this case, the peak near -105 ppm may be comprised by two distinct F species that are either bonded to boron or aluminum. To confirm this conclusion we carried out 19F{27Al}

and

19F{11B}

REDOR experiments on both samples Ge3 and B3. Figure 12

compares the standard rotor-synchronized echo signal, the signal obtained after recoupling the 27Al nuclei over a period of 6 rotor cycles (NTr = 0.24 ms), and the difference signal. It illustrates that the F1 and F1’ signals near -45 (-55) ppm remain almost unaffected by dipolar recoupling to either

11B

or

27Al,

indicating that these fluoride

species are remote from the network former species boron and aluminum. In contrast, the -105 ppm (F2, F2’) and the -138 (-145) ppm (F3, F3’) signals are significantly reduced in intensity upon 27Al dipolar re-coupling. Finally, dipolar recoupling to the 11B nuclei in the glass B3 also leads to a significant degree of intensity loss for the F2’ signal, consistent with the assignment that a least a part of it belongs to a boron-bonded fluoride species. Thus, in this particular glass the F2’ species giving rise to the peak near -104 ppm actually represents two overlapping fluoride species, BO3/2F- units as well as AlO4/2F- units, both of which are likely to be present. 24 ACS Paragon Plus Environment

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Figure 12: 19F{27Al} REDOR on the glasses Ge3 (top left) and B3 (top right) and REDOR on B3 (bottom), after 6 rotor cycles. The sharp signal is an artifact arising from the 19F background of the 3.2 mm probe used for these experiments. 19F{11B}

207Pb

CPMG-NMR. Figure 13 shows the static 207Pb CPMG-NMR spectra of the

six glasses under study. For all the glassy samples, extremely broad peaks typical of 207Pb NMR signals in glasses are observed, which are dominated by the magnetic shielding anisotropy. The spectra resemble those observed in numerous other Pb containing oxide glasses54-61 and could not be narrowed by the application of magic-angle spinning. This effect arises from extremely wide distributions of local magnetic shielding tensor values making it impossible to resolve spinning sideband manifolds. As a consequence, only average chemical shift values can be specified in these glasses, which are determined from the center of gravity, CG(207Pb), see Table 7. In the fluorogermanate glasses, CG appears to be compositionally independent within experimental error, near 2900 ppm vs. solid Pb(NO3)2. In the fluoroborate glasses, the value is slightly lower, near 2800 ppm, reflecting either bond covalency differences between Pb2+ ions bound to borate versus germanate ions, differences in the Pb coordination number or differences in the O/F ratio of the local Pb-ligand environment. For the glass B1, the signal is significantly displaced towards lower frequencies, suggesting that the average local environment of Pb differs 25 ACS Paragon Plus Environment


from that in the other glasses. Its average chemical shift value is, however, consistent with that (2425 ppm) given in ref. 28 for a glass of similar composition (50B2O3-45PbO5PbF2), taking the difference in chemical shift referencing, iso(relative to Pb(NO3)2 = 3491-iso (relative to Pb(CH3)4), into consideration. If there are significant differences in coordination number (as might be expected based on this shift), they could be detected via EXAFS measurements, to be conducted in the future. Substantial differences in the fraction of fluoride ions within the first coordination sphere of Pb might be detectable by 207Pb{19F}

REDOR experiments. While this latter experiment is particularly challenging

owing to the wide 207Pb chemical shift dispersion, such studies are under consideration in our laboratory.

Figure 13: 207Pb WURST-CPMG NMR spectra (Fourier transforms following echo addition) of the glasses under study.

Table 7: Center of gravity of the 207Pb wide line spectra of the glasses under study. CG (x103)/ppm

Sample

(±0.05 ×103 ) Ge1-0.2Yb

2.88

Ge2-0.2Yb

2.95

Ge3-0.2Yb

2.94

B1-0.2Yb

2.29

B2-0.2Yb

2.73

B3-0.2Yb

2.82


Page 26 of 32

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CONCLUSIONS In conclusion, we have conducted a comparative structural study of lead fluorogermanate and lead fluoroborate glasses, exploring the effect of compositional variations (PbF2 content and presence of Al2O3) upon their photophysical properties. The results are correlated with structural information obtained from EPR and multinuclear single- and double resonance solid-state NMR spectra. We identify various types of fluoride species including BO3/2F-, AlO4/2F2- and AlO4/2F23- units and species dominantly coordinated by Pb2+, whereas there is no evidence for Ge-F bonding. In the fluoroborate glass system, an increase in PbF2 content from 10 to 20 mole% has little influence upon the photophysical properties and the EPR spectra, suggesting no substantial changes in the O/F ligand distribution of the RE ions. A possible reason for this absence of composition dependence may be the extensive formation of B-F linkages detected by

11B

and

19F

MAS-NMR,

making this fluoride species unavailable for RE coordination. In contrast, the optical and EPR data obtained for the fluorogermanate system indicate that an increase in PbF2 content from 10 to 20 mole% affects the O/F ligand distribution of the RE ions significantly, as no Ge-F bonds are formed. The 27Al NMR data further suggest that effect of Al2O3 upon the photophysical properties of fluorogermanate glasses may be related to the formation of higher coordinated AlO4/2F2- and AlO4/2F23- species. These may favorably interact with the RE3+ ions, thereby contributing to an increased F/O ligand ratio in the local environment of the rare-earth species. In fact, the spatial proximity of the rare-earth species to aluminum is consistent with the observation of the 27Al Zeeman frequency in the HYSCORE experiments. In summary, the results of the present study offer a plausible structural rationale for understanding compositional changes in the photophysical properties in heavy metal oxyfluoride glasses.

ASSOCIATED CONTENT Supporting Information X-ray powder diffractograms, Differential scanning calorimetry plots, excited state lifetime measurements (experimental data and numerical fits), UV-vis absorption spectra, ESEEM and HYSCORE data on all glasses, 14.1 T 27Al MAS-NMR spectra of glasses in the system 40B2O3-10Al2O3-(50-x)PbO-xPbF2.



AUTHOR INFORMATION *corresponding author: email address [email protected] *corresponding author: email address [email protected] *corresponding author: email address [email protected] The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by FAPESP, grant number 2013/07793-6 and 2017/06649-0 (postdoctoral fellowship to C.D). E.C and W.F acknowledge CAPES fellowships.

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Table of Contents Figure


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Composition-Structure-Property Correlations in Rare-Earth-Doped

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