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Structural Studies of Fluoroborate Laser Glasses by Solid State NMR and EPR Spectroscopies Ruili Zhang, Marcos de Oliveira, Jr., Zaiyang Wang, Roger Gomes Fernandes, Andrea Simone Stucchi de Camargo, Jinjun Ren, Long Zhang, and Hellmut Eckert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11187 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 15, 2016

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The Journal of Physical Chemistry

Structural Studies of Fluoroborate Laser Glasses by Solid State NMR and EPR Spectroscopies Ruili Zhanga,b, Marcos de Oliveira,c Zaiyang Wanga, Roger Gomes Fernandesc, Andrea S. S. de Camargoc, Jinjun Rena*, Long Zhanga*, Hellmut Eckertc,d*

a

Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine

Mechanics, Chinese Academy of Sciences, Shanghai 201800, China b c

University of Chinese Academy of Sciences, Beijing 100039, China

Instituto de Física de São Carlos, Universidade de São Paulo (USP), C.P. 369, CEP 13560-970,

São Carlos, SP, Brazil d

Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstr. 30,

D-48149 Münster, Germany

ABSTRACT: The structure of glasses in the system (100-x)B2O3-xPbF2 (x = 30, 40, 50) and 50B2O3-(50-x)PbO-xPbF2 (x = 5, 10, 15, 20, 25, 20, 35, 40 and 45) has been studied by solid state NMR and EPR spectroscopies. Based on well as on

11

11

B and

19

F high resolution solid state NMR as

B/19F double resonance results, we develop a quantitative structural description on

the atomic scale. 19F NMR results indicate a systematic dependence of the fluoride speciation on PbF2 content: At low x-values, F- ions are predominantly found on BO3/2F- units, whereas at higher x-values, fluoride tends to be sequestrated into amorphous domains rich in PbF2. In addition, both pulsed EPR studies of Yb3+ doped glasses and photophysical studies of Eu3+ doped samples indicate a mixed fluoride/borate coordination of the rare-earth ions and the absence of nanophase segregation effects.

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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 have not only low phonon energies ascribed to the fluoride matrices but also high chemical and mechanical stability owing to the oxide-dominated framework structure. Therefore, they are considered to be a good choice as hosts for RE ions and, in fact, have turned out to be one of the most promising optical materials.6-10 In addition, glasses containing heavy metal oxides such as Pb, Cd and Bi, and lead borate, and fluoroborate glasses, in particular, have been studied in much detail because of their low phonon energies, high refractive indexes, low glass transition temperatures, high polarizabilities and good dispersal of RE ions.9-15. Other PbF2 containing RE-doped glass compositions have also shown promise for the preparation of RE-doped glass ceramics,16-19 with the perspective that the rare-earth ions are dispersed within the lattice of a PbF2 nanophase.19 In addition, lead fluoroborate glasses show fast anion conducting properties, making them interesting material for electrochemical devices.20-23 Despite this considerable interest, a fundamental understanding of these properties on the basis of the structural organization of these glasses is still missing. Solid state nuclear magnetic resonance (NMR) spectroscopy has proven to be a powerful tool in addressing such structural issues, especially for disordered materials, due to its well-proven ability to provide local structural information.24,25 Likewise, electron paramagnetic resonance of RE dopant ions is receiving increasing attention as a complementary structural probe, providing particular information on the local environment of the fluorescent dopant species.26-30 The present contribution reports a combined multinuclear NMR and EPR investigation of glasses in the model systems B2O3-PbF2 and B2O3-PbO-PbF2. These glasses were originally reported by Gressler and Shelby20,21 and could be promising candidates for the preparation of transparent ceramic lasers, based on the crystallization of PbF2 upon controlled annealing. Oneand two dimensional NMR experiments have been used to characterize the local environments of the boron, lead and fluoride species, whereas pulsed electron paramagnetic resonance (EPR)


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techniques have been employed for studying the rare-earth ion environments. In addition, the results have been correlated with the photophysical properties of glasses doped with Eu3+ ions. On the basis of the experiments mentioned above, we developed a quantitative structural description of this material. The results outline a general strategy for the structural elucidation of fluoride-containing glasses.

EXPERIMENTAL SECTION Sample Preparation and Characterization. A series of homogeneous glasses with nominal composition (100-x)B2O3-xPbF2 (x = 30, 40, 50) and 50B2O3-(50-x)PbO-xPbF2 (x = 5, 10, 15, 20, 25, 30, 35, 40, 45) were prepared by the conventional melt-quenching technique, using H3BO3, PbO, and α-PbF2 (Aladdin, purity > 99.9%) as starting materials. The starting materials were thoroughly mixed and heated in a platinum crucible at temperatures between 700 o

C and 800 oC, depending on the compositions. The melt was kept at this temperature for 15 min

to ensure homogenization, and it was subsequently rapidly poured onto a steel plate. The same procedure was used for the rare-earth (RE) doped oxyfluoride glasses which were prepared by adding rare-earth fluorides in excess 0.2~2 mole %. Glasses with molar compositions of (100-x) B2O3-xPbF2 (x = 30, 40, 50) are labeled as BFx. Glasses with molar compositions of 50B2O3(50-x)PbO-xPbF2 (x = 5, 10, 15, 20, 25, 30, 35, 40, 45) are labeled as BPFx. For these samples, the amorphous state was verified by X-ray powder diffraction (XRD) using filtered Cu Kα radiation in Bragg-Brentano geometry on a PANalytical Empyrean (Netherlands) diffractometer (diffraction angle scanning range 10o≤ 2θ ≤80o). Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were determined using a SII TG/DTA 7300 differential thermal analysis instrument (Seiko Co., Japan) at a heating rate of 10 K/min. Quantitative boron contents were determined by a

11

B single-pulse solid state NMR procedure, using NaBF4 as an

internal quantification standard. Because of better baseline and absence of phasing errors, the fluorine quantification experiments were carried out using back-extrapolated rotor-synchronized 19

F Hahn spin echo rather than single pulse spectra, using BaF2 as an internal quantification

standard. Details of the NMR quantification procedure are described in the Supporting Materials Section. Table 1 summarizes the glass compositions, and the percentages of boron and fluorine retained in the glass after cooling.


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Solid State NMR. All the NMR experiments were carried out at ambient temperature on a Bruker Avance III HD 500M spectrometer (11.7 T). 11B MAS experiments were acquired at the resonance frequency of 160.46 MHz, using a 4 mm MAS NMR probe operated at a spinning rate of 10 kHz. A 10o liquid pulse length of 0.46 µs was used. The recovery delay between scans was set to be 5 s and 16 s for full relaxation, respectively for the oxyfluoride glasses with and without Yb3+-doping. Chemical shifts are referenced to BF3·OEt2, using H3BO3 (=19.5 ppm) as a secondary reference. For all the nuclei detected, spin- lattice relaxation times were measured using the saturation recovery sequence. Line shape analysis and deconvolutions were done with the DMFIT software package.31 19

F MAS NMR spectra were carried out at 470.54 MHz, using a 2.5 mm probe operated at a

spinning rate of 25.0 kHz. 90o pulses of 2.35 µs length were used with a relaxation delay of 32 s, ensuring full signal recovery. Static 19F spin echo decay measurements were conducted using 90 ° and 180° pulses of 2.0 and 4.0 µs length, respectively, and an array of dipolar evolution times of 20, 30, 40, 50, 60, 80, 100, 150, 200, 250, 300, and 400 µs, respectively. Chemical shifts are referenced to CFCl3, using AlF3 (= -172.5 ppm) as a secondary reference. To investigate atomic connectivities and spatial proximities among the various 19F and 11B local environments, a number of

19

F{11B} and

11

B{19F} REDOR experiments were carried out,

using the pulse sequence of Gullion and Schaefer, operated at a spinning rate of 22~27 kHz.32 In the 11B{19F} REDOR experiments, the 180o pulse lengths for the 11B observe nuclei were 4.8 µs for the four-coordinated boron species. The relaxation delay was 8 s. The was 4.7 µs. For

19

19

F 180o pulse length

F{11B} REDOR experiments, typical 180o pulses of 4.7 µs and 4.8 µs length

were employed for

19

F and

11

quantitative information, the

B, respectively, and a relaxation delay of 16 s was used. To get 11

B{19F} REDOR dephasing curves were simulated with a

heteronuclear two-spin model using the SIMPSON package33 and these simulations were based on the actual experimental values of the parameters used in the pulse sequence. To explore the possibility of detecting F-bonded borate species,

11

B{19F} 2D heteronuclear correlation

(HETCOR) experiments were carried out. These experiments were done at a spinning frequency of 15 kHz. Good spin-lock and Hartmann Hahn matching conditions were found at nutation frequencies of 71.9 and 62.5 kHz for 11B (non-selective) and 19F, respectively, and the 11B power level was subjected to a linear ramp down to a nutation frequency of 36.0 kHz. 256 scans were


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accumulated with a relaxation delay of 8 s, using contact times of 200 and 1000 µs, and with 14 increments of the pre-Hartmann-Hahn contact evolution time in steps of 10 µs. Reverse 19F{11B} experiments were also attempted but found to be unsuccessful, possibly because of the weaker homonuclear 11B-11B dipole-dipole interactions. Static 207Pb Carr-Purcell-Meiboom-Gill (CPMG)34,35 spectra were obtained at 104.6 MHz using a 4 mm probe. The 90o and 180o pulse lengths were 5µs and 10µs, respectively, and a relaxation delay of 10 s was used. The acquisition time of an echo (2τ) was 1.20 ms, the pulse separation between the first two pulses (τ1) was 20 µs. The dead times (τ2) were set to 20 µs, more than 400 scans were accumulated with a recycle delay of 10 s. Full trains of typically 12 echoes were acquired and were subsequently Fourier transformed, resulting in spikelet patterns with a spikelet separation of 5882 Hz. To overcome probe bandwidth limitations, the carrier frequency was systematically varied in 35.278 kHz steps, and the resulting spikelet patterns were superimposed upon each other, resulting in a composite line shape reflecting uniform excitation over the entire powder pattern. The envelope of the spikelet pattern forms the static anisotropic powder pattern, whose form is dominated by the magnetic shielding anisotropy, with a distribution of principal tensor values. Solid lead nitrate was used as a secondary external reference for the

207

Pb spectra, the isotropic chemical-shift being -3491 ppm relative to

tetramethyl lead (TML) at 298 K.36,37 Solid State EPR. Pulsed solid-state EPR experiments were carried out on Yb3+-doped (0.2 mol %) samples at 6.5 K on an E-580 BRUKER ELEXSYS X-band EPR spectrometer. Due to very fast spin-spin relaxation no electron spin echo was observable at temperatures above 10 K. Electron spin echo envelope modulation (ESEEM) spectra were obtained at external field strengths of 700 and 900 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 100 ns or 120 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.40 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


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component. Following further apodization and zero-filling, the oscillating signal was Fouriertransformed, resulting in the ESEEM spectrum. The echo detected absorption spectra were recorded using the three-pulse sequence. The integrated echo intensities were measured as a function of the magnetic field strength over a range of 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. 2D-HYSCORE spectra were obtained at an external magnetic field strength of 700 mT using the four-pulse sequence (tp) - τ - (tp) - t1 - (2tp) - t2 - (tp) – echo.38,39 The echo intensity was measured as a function of the evolution times t1 and t2, which were incremented in steps of 12 ns from the initial values of 300 ns. Pulses of tp = 8 ns length for the π/2 pulse and 2tp = 16 ns length for the π pulse were used to record a 128×128 matrix. Following further apodization and zero-filling (to 512×512 points), the oscillating signal was Fourier-transformed in both dimensions, resulting in the two-dimensional HYSCORE spectrum. A 4-step phase cycle was used to eliminate unwanted coherences. In the HYSCORE sequence the effect of the additional π pulse is to transfer nuclear coherence between the two electron spin manifolds, α (spin-up) and β (spin-down), allowing the correlation of nuclear transitions from different manifolds.30 These correlations manifest themselves as non-diagonal cross-peaks at (να , νβ), (νβ , να) and (-να , νβ), (-νβ , να), respectively in the (+,+) and (-,+) quadrants of the 2D-spectrum.38,41 In the limit of a weak hyperfine interaction (A < 2νI), the contributions with positive phase modulation dominate and the crosspeaks appear in the (+,+) quadrant.38,42,43 In the opposite limit of a strong hyperfine interaction (A > 2νI), the contributions with negative phase modulation dominate and the cross-peaks appear mostly in the (-,+) quadrant. Near the cancellation condition (Aiso ~ 2νI), the cross-peaks in the 2D-HYSCORE spectrum have comparable intensities in both quadrants.38,42 All data processing and lineshape simulations and fits to ESEEM and HYSCORE spectra were done using the Easy spin software.44 Photophysical Characterization. The fluorescence spectra and excited state lifetimes of Eu3+-doped samples were recorded by a high resolution FLS920 spectrofluorometer (Edinburgh Instruments), using a Xe lamp as the excitation source. The scanning step of 1 nm was used to measure emission spectra. All measurements were carried out at room temperature.


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RESULTS AND DISCUSSION Glass Compositions. Large deviations between nominal and experimental glass compositions are a serious concern in the preparation of fluoroborate glasses. This issue was addressed in the present contribution by detailed fluoride and boron quantification experiments using internal mass calibration standards. As summarized in the Supporting Materials Section, the synthesis of pure BFx glasses is seriously handicapped by the formation of volatile BF3, leading to weight losses during synthesis in the range of 13%. While such gross mass losses are not observed for the BPFx glasses, the fluorine content of these glasses is also clearly reduced to about 60-80% of the original quantity batched. As previously discussed15,23 this fluoride loss most likely occurs through F/O exchange via the melting atmosphere. Obviously these losses must be taken into consideration when discussing the experimental NMR results.

Table 1. Glass Compositions, Residual Content of B and F Quantified by NMR, Measured Weight Loss. compositions Sample label B2O3 PbO PbF2

Residual content quantified by NMR

Weight loss

B /%

F /%

Expa /%

weightb /%

BF30

70

0

30

84.8

23.4

13.2

13.0

BF40

60

0

40

84.0

27.6

12.6

12.6

BF50

50

0

50

88.5

36.9

10.2

13.1

BPF0

50

50

0

98.8

0

0.3

2.2

BPF5

50

45

5

96.3

79.9

1.1

1.8

BPF10

50

40

10

95.9

58.9

2

2.2

BPF15

50

35

15

95.4

64.3

2.4

2.1

BPF20

50

30

20

94.8

58.1

3.3

4.8

BPF25

50

25

25

91.7

51.5

4.9

3

BPF30

50

20

30

98.0

50.1

4.2

4.8


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a

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BPF35

50

15

35

96.8

49.0

5.1

7.5

BPF40

50

10

40

93.6

45.0

6.8

10.4

BPF45

50

5

45

87.2

42.8

9.1

10.5

Calculated according to the NMR quantification experiment results.

b

Determined by calculating the difference between expected glass weight according to the nominal compositions and final obtained glass weight. n.m. Not measured

High-Resolution 19F MAS NMR and 11B MAS NMR. Figure 1a presents the 19F MAS NMR spectra of the BF and BPF system. The results of the line shape deconvolution are shown in Table 2. Representative fits of sample BF40 are shown in Figure S5 (see Supporting Materials Section). In accordance with previous assignments of related fluoroborate glasses,15 the signal around -48 ppm is assigned to

19

F species interacting

predominantly with Pb2+, while the signal near -110 ppm can be ascribed to 19F nuclei in BO3/2Funits, as confirmed by 19F{11B} REDOR and 11B{19F} HETCOR (see below). In the BF system, the intensity ratio of these two species changes monotonically with increasing Pb/B ratio. In the BPF system, where the Pb2+/B3+ ratio is kept constant, the fluorine speciation remains approximately constant. These results indicate that the F- distribution is mainly the result of competitive attraction of fluoride by Pb2+ and B3+. Figure 1b shows the 11B MAS NMR spectra. Three- and four-coordinated boron units can be easily resolved. The lineshape of the threecoordinate (B(III)) species is characterized by second-order quadrupolar broadening. For the four-coordinate (B(IV)) 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 3 summarizes the lineshape parameters. In the BFx system N4, the fraction of four-coordinate boron atoms, increases with increasing x, whereas in the BPF system N4 remains at an approximately constant level of 0.60 ± 0.02.


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Figure 1. High-resolution 19F MAS NMR and 11B MAS NMR spectra of the BF and BPF system. (a) 19F MAS NMR spectra. (b) 11B MAS NMR spectra. Spinning sidebands are indicated by asterisks. Table 2. Deconvolutions of the 19F MAS-NMR Spectra of the Glasses Studied. F2

F1 Sample

fraction (±1%)

BF30

30.9

-48.4

29.2

69.1

-116.3

31.1

BF40

56.3

-46.8

28.8

43.7

-112.3

29.6

BF50

77.0

-45.9

28.8

23.0

-109.7

30.4

BPF5

76.4

-54.0

31.2

23.6

-109.8

30.8

BPF15

74.8

-49.9

31.1

25.2

-110.4

30.8

BPF25

72.2

-47.8

30.3

27.8

-110.7

29.8

BPF45

74.7

-44.0

28.0

25.3

-111.3

28.5

δCSiso FWHM (±0.5 ppm) (±0.5 ppm)

fraction (±1%)

δCSiso FWHM (±0.5 ppm) (±0.5 ppm)


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Table 3. 11B Lineshape Parameters Obtained from the Spectra of the Glasses Studied. B(III)

a b

B(IV)

Sample

fraction (±1%)

δCSiso (±0.5ppm)

ηQ (±0.05)

Cq (±0.05MHz)

fraction (±2%)

δCSiso (±0.5ppm)

BF30

51.4

17.3

0.20

2.57

48.6

0.3

BF40

42.7

18.0

0.21

2.57a (2.66)b

57.3

0.8

BF50

39.7

18.6

0.22

2.56 (2.66)

60.3

1.2

BPF0

49.4

18.9

0.20

2.54

50.6

1.4

BPF5

46.3

18.9

0.20

2.54

53.7

1.4

BPF10

43.3

18.8

0.21

2.55

56.7

1.3

BPF15

41.3

18.6

0.20

2.54

58.7

1.3

BPF20

41.8

18.6

0.20

2.55

58.3

1.3

BPF25

40.3

18.2

0.20

2.55

59.7

1.0

BPF30

40.3

17.9

0.21

2.56

59.7

0.6

BPF35

40.0

18.0

0.20

2.55

60.0

0.7

BPF40

39.0

18.0

0.21

2.55

61.0

0.8

BPF45

39.1

18.3

0.21

2.56

60.9

1.0

Values obtained by fitting single pulse spectra. Values obtained by MQMAS.

While TQMAS experiments (see Supporting Materials Section), produce better resolved spectra, no new species can be identified. The 2

1/2

quadrupolar effect SOQE=CQ(1 + η /3)

11

B isotropic chemical shift and the second-order

deduced from these data are summarized in Table 3.

To characterize the strength of boron-fluorine interaction and bonding,

11

B{19F} REDOR

experiments were carried out. Results obtained on representative samples of the BFx and BPFx systems are summarized in Figure 2. Part (a) shows the comparison between and

11

11

B{19F} REDOR

B spin echo spectra of sample BF40. Clearly, the signals attributed to B(IV) units are

significantly attenuated by 19F irradiation, indicating that fluorine atoms are in close proximity to them. In contrast, the B(III) units are much less affected by the dipolar recoupling with the

19

F

nuclei, suggesting that these units are more remote from fluorine atoms. The quantitative


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analysis of the results indicates that only some of the B(IV) units, but none of the B(III) units have direct connectivity with

19

F. Figures 2b-d show full REDOR curves, obtained by

systematically increasing the dipolar evolution time. The dashed curves correspond to SIMPSON simulations assuming a two spin system involving one 11B and one neighboring 19F nucleus. The REDOR curve obtained for the B(IV) units is a superposition of a component arising from 11Bnuclei dephasing rapidly because they are F-bonded (and thence are close to

19

F, resulting in

strong dipolar coupling) and from 11B nuclei dephasing much more slowly because they are not F-bonded (weak dipolar coupling). This superposition results in an apparent plateau in the Figures 2b and d. The B-F bond dominates the initial fast dephasing data part, while the non-F bonded BO4 units dominate the slower part. In this manner, the

11

B{19F} REDOR experiment

allows a quantitative differentiation between the BO3F and BO4 species by comparing the experimental data with corresponding simulation results. The simulation parameters are shown in Table 4. This table also includes dipolar second moments extracted by a parabolic analysis of the initial part of the experimental REDOR curves (∆S/S0 ≤ 0.2), using the formula ∆S/S0 = 4/3π2 × (NTr)2 M2(B-F) employed in many other REDOR applications to glasses.45,46 For all of the non-F-bonded B(IV) and all the B(III) units the strength of the REDOR effect is seen to increase with increasing F content of the glass (see Table 4), reflecting the increased statistical probability of 19F to be present within higher coordination spheres with increasing x.


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Figure 2: 11B{19F} REDOR results for (100-x)B2O3-xPbF2 and 50B2O3-(50-x)PbO-xPbF2 glasses: (a) comparison between spectra obtained with and without recoupling the 11B-19F magnetic dipolar interaction and 11B spin echo spectra of sample BF40, using a mixing time of 0.148 ms. (b) 11B{19F} REDOR curves for B(III) and B(IV) species of sample BF40. The dashed lines represent two-spin SIMPSON simulations of the REDOR curves. (c) 11B{19F} REDOR curves for three-coordinated boron sites measured for various glasses. (d) 11B{19F} REDOR curves for fourfold-coordinated boron sites measured for various glasses. The solid curves represent two-spin SIMPSON simulations of the REDOR curves.


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Table 4. Simulation Results of the 11B{19F} REDOR Dephasing Curves for the (100-x)B2O3xPbF2 and 50B2O3-(50-x)PbO-xPbF2 Glasses. B(III)

B(IV)

Sample

B-F average distance (Å)

M2(B-F) (106 rad2s−2)

BO3/2F fraction (%)


BO4/2 fraction (%)


BF30

3.4

10.5

20

1.5

80

3.5

BF40

3.15

17.6

20

1.5

80

3.4

BF50

3.1

20.7

16

1.5

84

3.3

BPF5

3.9

3.7

4

1.6

96

4.3

BPF25

3.15

14.5

16

1.5

84

3.4

BPF45

3.1

19.8

19

1.5

81

3.3

Figure 3 shows representative dipolar recoupling on the

19

19

F{11B} REDOR data. Part (a) shows the effect of

F signal intensities at a fixed dipolar mixing time. The

contributing to the -110 ppm resonance interact much more strongly with

11

19

11

B

F nuclei

B than those

contributing to the -48 ppm signal. The quantitative analysis of these data is complicated owing to the quadrupolar nature of the boron nuclei and the fact that B(III) and B(IV) have different nutation frequencies, resulting in different recoupling efficiencies at any chosen

11

B nutation

frequency. The analysis is further compromised by significant spinning sideband overlap of the two

19

F resonances. Nevertheless, the data allow the conclusion that all of the F species

contributing to the -110 ppm peak are boron bonded, whereas those contributing to the -48 ppm peak are more remote from 11B.


13


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Page 14 of 33

Figure 3. 19F{11B} REDOR results for (100-x)B2O3-xPbF2 and 50B2O3-(50-x)PbO-xPbF2 glasses. Such REDOR data are obtained by fitting the spectra assuming the sidebands are symmetric and separating them. (a) Comparison between spectra obtained with recoupling the 11 19 B- F magnetic dipolar interaction and 11B spin echo spectra of sample BF40, using a mixing time of 0.074 ms. (b) 19F{11B} REDOR curves for sample BF40. (c) 19F{11B} REDOR curves for sample BPF25.


14

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Based on these results we can determine the number of boron-fluorine bonds from both experiments independently. The number of B-F bonds present in these glasses as deduced from 19

F NMR amounts to f(-110 ppm) × 2x, where f(-110 ppm) is the fractional contribution of the

19

F NMR signal at -110 ppm and x is the PbF2 content in mole%. This number can be compared

with the number of F-B bonds deduced from

11

B NMR, which is given by mole % B2O3 × f

(REDOR) × N4 where f (REDOR) is the fractional contribution of the B(IV) species showing a very rapid

11

B{19F} REDOR dephasing at short evolution times (see Table 4). As indicated in

Table 5 the two results are quantitatively consistent with each other, when the boron and fluorine losses incurred during the synthesis of the glasses are taken into consideration. Complementary results regarding medium-range order can be obtained from 2D-11B{19F} HETCOR NMR, indicating which type of 11B species is being cross-polarized from which type of

19

F species. Figure 4 shows representative results obtained on sample BF40. When the short

contact time of 200 µs is used, correlations are observed only between the most strongly coupled (i.e., directly bonded) nuclei. Figure 4a impressively demonstrates the selective correlation of the four-coordinated BO3/2F units with the

19

F resonance near -110 ppm, again confirming that the

latter signal can be assigned to B-bonded F species. In contrast, at the longer contact time of 1 ms, the HETCOR spectrum demonstrates the participation of 19F species contributing to the -48 ppm signal to the cross-relaxation process as well (Figure 4b). This latter result shows that these fluorine species are also dipolar-coupled to the boron nuclei, consistent with the

19

F{11B}

REDOR results. Furthermore, we can conclude that the B(III) units are in dipolar contact with both fluoride species, indicating the absence of nanophase separation.

.


15


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Page 16 of 33

Figure 4. 11B{19F} HETCOR experiment of sample BF40. The mixing time is 0.2 ms (top) and 1ms (bottom), respectively.

Table 5. B-F Bond and F-B Bond Concentrations, Dipolar Second Moments, M2(F-F) and Center of Gravity CG(207Pb) of the 207Pb Resonance in the Glasses under Study. M2(F-F) a b (106 rad2s−2) N(B-F) N(F-B) CG(207Pb) Samples ± 10% (10-2mol) (10-2mol) ±10 ppm F1 F2 BF30

5.9

5.0

n.m.

n.m.

n.m.

BF40

5.9

5.0

n.m.

n.m.

n.m.

BF50

4.3

4.9

n.m.

n.m.

-2079

BPF0

n.m.

n.m.

n.m.

n.m.

-896

BPF5

1.1

1.3

97

97

-1066

BPF25

4.4

4.7

353

184

-1911


16

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BPF45

5.1

5.9

553

274

-2071

PbO

n.m.

n.m.

n.m.

n.m.

1971

PbF2 n.m. n.m. n.m. n.m. 11 11 19 From B, B{ F} REDOR NMR, and the boron quantification results b From 19F NMR, considering the fluorine quantification results n.m., not measured

-2631

a

Figure 5 summarizes the results from

19

F static Hahn spin echo decay measurements. In

part (a) the Fourier transforms for sample BPF45 are shown at different dipolar mixing times. The two signals centered at about -47 and -112 ppm are the same species observed in the MASNMR spectra of Figure 1. Despite the loss in resolution it was possible to analyze the spin echo intensity decays of these two species via two separate Gaussian lineshape components, resulting in two distinct Gaussian spin echo decay curves (Figure 5b) according to the expression I(2t1)/I0 = exp{-(2t1)2M2(F-F)/2}. previously used for the dipolar structural analysis of glasses.47 In this expression, M2(F-F) denotes the dipolar second moment characterizing the homonuclear

19

F-19F magnetic dipole-dipole

interactions. Figure 5c shows a plot of M2(F-F), as a function of PbF2 content for three glasses investigated by this method. The data are compared with the behavior predicted for a statistical distribution in space, based on a truncated PbF2 lattice model. In this model the fluoride ions are arranged on the anionic sites of a β-PbF2 lattice, but with a reduced occupancy, taking the actual fluoride contents of the glasses (as determined by NMR) into account. The model assumes a statistical distribution of F- ions over these sites, with M2 values depending linearly on occupancy. Note that the M2(19F-19F) values for the Pb-bonded species lie above the value predicted from the truncated lattice model, indicating a certain extent of F clustering. In contrast, the data for B-bonded F- species fall below the value predicted from a truncated PbF2 lattice model indicating that these species are in environments lower in F content than predicted by the average glass composition in which the F atoms are assumed to be statistically distributed.


17


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Page 18 of 33

Figure 5. (a) 19F static Hahn spin echo Fourier Transforms of the glasses BPF45 at different dipolar evolution time (20, 30, 40, 50, 60, 80, 100, 150, 200, 250, 300, and 400 µs, respectively). (b) 19F static Hahn spin echo amplitudes of the glasses BPF45 as a function of dipolar evolution time, where F1 and F2 denote the species at -47 ppm (Pb-bonded) and -112 ppm (B-bonded F). (c) M2(19F-19F) values for the various different fluorine species in BPF5, BPF25, and BPF45. The solid line is the expected behavior based on a statistical truncated lattice model of PbF2. This model was calculated by arranging the F atoms within the glass (based on the NMR quantification) on the anionic sites of a PbF2 lattice.


18

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207

Pb CPMG-NMR. Figure 6 shows the static

207

Pb CPMG-NMR spectra of BFx and

BPFx glasses. Spectra of crystalline PbO and PbF2 are also included. For the model compounds, isotropic chemical shifts of -2631 ppm and 1971 ppm are determined for PbF2 and PbO, respectively in good agreement with the literature.48-50 For all the glassy samples, extremely broad peaks typical of

207

Pb 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 glasses51-63 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). This average isotropic chemical shift shows a clear evolution from higher towards lower values with increasing PbF2 content, consistent with an increase of the F/O ratio in the first coordination sphere of Pb. No PbO-like or PbF2-like signals are observed, indicating the absence of cluster domains with local Pb environments resembling those in these binary phases.


19


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Page 20 of 33

Figure 6. Static 207Pb CPMG-NMR spectra of the glasses of crystalline PbO and PbF2. PbF2 was measured by static Hahn echo.

Pulsed EPR Spectroscopy on 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. The 2F5/2 term has little effect on the magnetic properties due to the considerable energy difference between these two levels (10000 cm-1 for the free Yb3+ ion). Owing to the low dopant concentrations used (0.2 mol%), inter-atomic dipole interactions can be neglected. Figure 7a shows the echo detected field sweep (EDFS) EPR spectra for the BPFx glasses, with x = 0, 5, 25 and 45. The spectra are composed of broad asymmetric lines. No specific features attributable to 171

Yb and

173

Yb (natural abundances 14.3% and 16.1% respectively) nuclear hyperfine


20

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interaction can be discerned. No electron spin echo is detected at zero field, consistent with the absence of rare-earth clustering.26 Most likely the spectral lineshape is dominated by the ganisotropy and site heterogeneity for the Yb3+ ions. The position of the EDFS resonance can be qualitatively determined by the center of gravity CG of the spectra. Due to unavailability of higher magnetic fields the whole spectra couldn’t be measured, and the CG values consider only the observed spectral window available. A plot of CG as a function of the [PbF2] content is shown in Figure 7b. Except for the sample BPF0, we notice a systematic shift to higher magnetic fields for increasing PbF2 content. This up-field shift for fluoride richer samples has already been observed previously for aluminum-fluoride-phosphate glasses.28

Figure 7 (a) EDFS spectra for the BPFx glasses, with x = 0, 5, 25 and 45 and (b) their respective center of gravity as a function of the PbF2 content in the glass composition.


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Page 22 of 33

Figure 8 – Three-pulse ESEEM spectra for glasses in the series 50BPFx, with x = 0, 5, 25, 45. The spectra were recorded at magnetic field strengths of 550 mT (a), 700 mT (b) and 900 mT (c). The τ parameter (first delay in the pulse sequence) was selected in order to avoid blind spots in the spectra. The values are 100 ns (a), 120 ns (b) and 100 ns (c). Resonances are marked with the corresponding nuclear species.


22

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Figure 8 shows the ESEEM spectra for the BPFx glasses, with x = 0, 5, 25 and 45, measured at magnetic field strengths of 550 mT (Figure 8a), 700 mT (Figure 8b) and 900 mT (Figure 8c). Clearly, the set of spectra reveal resonances at the Larmor frequencies of 207

Pb and

19

10

B,

11

B,

F (peaks are marked in the Figure and the frequencies are shown in Table 6),

suggesting that these isotopes are interacting with the electron spins residing in the 4f orbitals of the Yb3+ ions. The indicated resonances lie in the weak-coupling limit, i.e., the hyperfine coupling constants (in Hz) are small compared to the nuclear Zeeman frequencies. Each set of ESEEM spectra in Figures 8a-c were measured under identical conditions, which allow us to compare the relative peak intensities of the observed resonances. The B/Pb ratio in the glass compositions remains constant within this series. Accordingly, the relative intensities of the 10B, 11

B and

207

Pb resonances are similar when comparing ESEEM spectra measured at same

magnetic fields for a given τ value. On the other hand, the relative intensity of the 19F resonance increases with increasing PbF2 concentration in the expected manner. Also, this signal is not detected in the sample with the lowest PbF2 concentration (x = 5). Table 6– Nuclear Zeeman Frequencies for the Isotopes Observed in the ESEEM and HYSCORE Experiments at Different Magnetic Fields. Magnetic field υ(10B) /MHz υ(11B) /MHz υ(207Pb) /MHz υ(19F) /MHz (mT) 550

2.52

7.51

4.97

22.04

700

3.20

9.56

6.32

28.05

900

4.12

12.30

8.13

36.07

Further insight into the hyperfine couplings between the unpaired electron in the Yb3+ ion and the neighboring nuclei is available from the HYSCORE spectra. Figure 9 summarizes the results. All resonances were observed in the (+,+) quadrant of the spectra. For all spectra in Figure 9 diagonal peaks corresponding to the Zeeman frequencies of

10

B,

11

B and

207

Pb are

observed. The concentration of these signals in the diagonal region indicates that these species are weakly interacting with the Yb3+ ion, i.e., |A/2|

Structural Studies of Fluoroborate Laser Glasses ... - ACS Publications

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