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GeAs thiophosphate glasses: properties and NMR spectroscopy
Journal of Non-Crystalline Solids 263&264 (2000) 117±122
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GeAs thiophosphate glasses: properties and NMR spectroscopy B.G. Aitken *, R.E. Youngman SP-FR-05, Corning Incorporated, Corning, NY 14831, USA
Abstract Stoichiometric GeAsP sulphide glasses containing 50±90 mol% GeS2 and 4±25 mol% P2 S5 have been investigated to determine the compositional and structural dependence of physical properties. The latter vary monotonically with increasing P2 S5 concentration: at constant GeS2 content, the thermal expansion coecient increases, whereas refractive index, isokom temperatures and density decrease as P2 S5 replaces As2 S3 . The introduction of P is marked by absorption bands in the 900±1400 cmÿ1 range that are absent in P-free glasses. The compositional dependence of optical properties and molar volume indicates that the dominant P species in these glasses is the tetrahedral S¸PS3=2 group. This interpretation of the local P environment is con®rmed by 31 P MAS NMR spectral data, which further indicate that the latter species tend to have AsS3 groups as next-nearest neighbors. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Rare-earth doped GeAs sulphide glasses are of interest for the fabrication of a variety of photonic devices, including 1.3 lm optical ampli®ers and mid-IR lasers [1]. Most of these devices require the active glass to be made in the form of an optical ®ber. The latter in turn requires that the core of such a ®ber be clad by a chemically and physically compatible glass with smaller refractive index. P additions to GeAs sulphide glasses have been found to have the desired eect of decreasing the refractive index [2]. Previous work has demonstrated glass formation in the limiting ternary Ge± As±S, Ge±P±S and As±P±S systems [3±7]. This study was undertaken to determine the extent of
* Corresponding author. Tel.: +1-607 974 3111; fax: +1-607 974 3675. E-mail address: [email protected] (B.G. Aitken).
glass formation in the broader quaternary system, as well as the compositional and/or structural dependence of physical properties relevant to the ultimate goal of ®ber fabrication. 2. Experimental GeAsPS glasses were synthesized from 36 g mixtures of elements with P 99.9995% purity (metals basis) that were loaded into 10 mm ID fused silica ampoules under N2 . Prior to batching, the ampoules were etched in 5% HF:5% HNO3 , rinsed in deionized water and then dried at about 1000°C. The ®lled ampoules were evacuated to 10ÿ6 Torr, ¯ame sealed and then heated to 800± 875°C for at least 24 h in a rocking furnace. Cylindrical 15 cm long glass rods were formed by quenching the hot ampoules in water and subsequently annealing near the glass transition temperature (Tg ).
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 6 2 7 - 4
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Density was measured in water to 0.001 g/cm3 using the Archimedes method. Tg was determined by dierential scanning calorimetry at a heating rate of 10°C/min with a precision of 2°C. The viscosity in the 105 ±109 P range was determined by the parallel plate technique [8] using fused silica plates with an experimental error of 2°C. Thermal expansion was measured from room temperature to 200°C to 0.1 ppm/°C by dilatometry using Al2 O3 as a reference. Refractive index was measured to 0.01 by the apparent depth method at 589 nm with 5 mm thick samples. Optical absorption was measured using a spectrophotometer (Cary 5G for visible/near IR and Mattson Cygnus for IR). 31 P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were collected using a 5 mm MAS NMR probe (Chemagnetics) and In®nity console, in conjunction with a 4.7 T superconducting magnet (80.4 MHz resonance frequency). Samples were powdered and packed into ZrO2 rotors. The NMR data were collected at a spinning rate of 12 kHz with single pulse excitation (pulse width 3.3 ls) and recycle delays of 600±900 s. The chemical shifts were referenced to an external solution of 85% H3 PO4 .
3. Results Stoichiometric GeAsPS glasses, i.e., glasses whose chemical composition lies in the GeS2 ± As2 S3 ±P2 S5 plane, can be formed at modest quenching rates from all compositions save for small regions in the immediate vicinity of GeS2 and P2 S5 . The glass samples investigated in this study are all relatively Ge-rich, containing P 50% GeS2 and their physical properties vary monotonically with composition (Table 1). Within the studied range, the refractive index (nD ) varies from 2.18 to 2.48. Upon substituting P2 S5 for As2 S3 , nD decreases by about 0.006 per mol% P2 S5 , but shows no decrease within errors of measurement on substituting P2 S5 for GeS2 (Fig. 1). The glass samples range in colour from yellow to reddish orange, corresponding to a shift of the visible absorption edge (kvis wavelength where the transmission of a 2 mm thick sample is 50% of that at 1.0 lm) from 500 to 560 nm. As with nD , kvis is a function solely of As content, shifting to shorter wavelength as P2 S5 replaces As2 S3 at a constant GeS2 concentration, but remaining unchanged when P2 S5 is substituted for GeS2 . Whereas P-free
Table 1 Physical properties of stoichiometric GeAsP sulphide glasses % GeS2 (mol%)
% P2 S5 (mol%)
nD
kvis (nm)
Tg (°C)
Ts (°C)
a (ppm/°C)
q (g/cm3 )
Vmol (cm3 /mol)
90 83.3 83.3 83.3 83.3 83.3 78.3 72.7 70 66.7 66.7 66.7 66.7 66.7 60 55 50 50 50 50 50
10 4.2 8.3 10 12.5 16.7 4.3 9.1 10 5 10 14.3 20 33.3 10 10 5 10 15 20 25
2.23 2.29 2.26
501 512 508 506 508 506 520 526 523 535 527 528 524
353 331 337
485 466 458
15.8 15.6 16.3
315 300 317 280 270 278 265 282 257 218 253 252 248 239 228 225 218
433 433 421 395 372 373 370 360 337 350
17.2 18.5 16.5 18.8 18.3 17.8 18.6 19.6 20.9 27.7 19.2
331 324 311 303 293
21.3 21.4 22.8 24.0
2.641 2.801 2.722 2.688 2.636 2.558 2.823 2.767 2.767 2.874 2.783 2.701 2.610 2.373 2.814 2.841 2.948 2.855 2.781 2.702 2.619
16.18 16.09 16.06 16.06 16.09 16.09 16.04 15.93 15.90 15.89 15.87 15.90 15.87 16.07 15.82 15.76 15.73 15.76 15.71 15.70 15.74
2.24 2.22 2.31 2.32 2.24 2.38 2.33 2.32 2.29 2.20 2.37 2.41 2.41 2.40 2.35 2.33 2.31
536 541 554 547 538 533 527
B.G. Aitken, R.E. Youngman / Journal of Non-Crystalline Solids 263&264 (2000) 117±122
Fig. 1. Variation of nD of stoichiometric GeAsP sulphide glasses (1) on substituting P2 S5 for As2 S3 at constant GeS2 concentrations of 83, 67 and 50 mol% and (2) on substituting P2 S5 for GeS2 at 0 and 18 mol% As2 S3 . The lines are linear ®ts to the data.
GeAs sulphide glasses have an IR cuto near 850 cmÿ1 , several absorption bands at about 920, 1100, 1190 and 1370 cmÿ1 , whose intensity increases with increasing P2 S5 concentration, are present in the IR spectrum of P-containing glasses. The samples are all low temperature glasses, with Tg ranging from 200°C to 350°C and the softening point (Ts the temperature at which the glass viscosity equals 107:6 P) varying from 290°C to 480°C. Both Tg and Ts decrease with increasing P content. The change in Ts is largest for the case where P2 S5 replaces GeS2 , corresponding to a decrease of about 7.5°C per mol% P2 S5 . The viscosity decrease reduces to about 2.5°C per mol% P2 S5 upon replacing As2 S3 with P2 S5 . The thermal expansion coecient (a) of the studied glasses ranges from 15 to 28 ppm/°C, with its compositional dependence being the inverse of that of the viscosity: a increases with increasing P concentration, with the rate of increase being greater as P replaces Ge. The density of these glasses varies from 2.3 to 3.0 gm/cc and, as expected from the smaller atomic mass of P, decreases with increasing P content. The molar volume (Vmol , where the latter is calculated on a constant atom basis using molecular
119
formulae of the form Gex Asy Pz S1ÿxÿy ÿz ) decreases by about 0.01 cm3 /mol% P2 S5 as P2 S5 replaces GeS2 at constant As2 S3 content (Fig. 2). However, its decrease is an order of magnitude less as P2 S5 is substituted for As2 S3 . Finally, the durability of stoichiometric GeAsP sulphide glasses decreases with increasing P content. After exposure to ambient humidity, glasses containing in excess of about 12% P2 S5 develop an opaque yellow hydrated surface layer, which is con®rmed by the appearance of peaks near 0 ppm in the 31 P MAS NMR spectra. Portions of the same P-rich glass samples retain a pristine surface when stored in a desiccator. 31 P NMR spectra for two series of these samples at 50% GeS2 and 10% P2 S5 are provided in Figs. 3 and 4, respectively. The data for the former series are aected by microstructural changes as a function of substituting P2 S5 for As2 S3 . The spectrum of the 5% P2 S5 glass has two features: a smaller symmetric peak at 116 ppm and a more intense and asymmetric peak at about 74 ppm. The latter
Fig. 2. Compositional dependence of Vmol of stoichiometric GeAsP sulphide glasses (1) on substituting P2 S5 for As2 S3 at 83, 67 and 50 mol% GeS2 and (2) on substituting P2 S5 for GeS2 at 0% As2 S3 . For comparison with the latter series, data for stoichiometric GeAs sulphide glasses from Ref. [10] are plotted as a function of As2 S3 concentration. The lines are linear ®ts to the data. Error bars are equal in size to or smaller than the data symbols.
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Fig. 3. 31 P MAS NMR spectra of (GeS2 )50 (As2 S3 )50ÿx (P2 S5 )x glasses: (a) x 5, (b) x 10, (c) x 15, (d) x 20 and (e) x 25 mol% P2 S5 . The data have been referenced to 85% H3 PO4 . Three Gaussian peaks are indicated by the dashed curves in (a). A vertical line has been drawn at 74 ppm as a guide to the eye.
feature can be deconvoluted into two overlapping Gaussian peaks centered at 76 and 63 ppm. The gradual replacement of As2 S3 by P2 S5 does not aect the chemical shifts of these peaks, but does change their relative intensities. The resonance at 116 ppm increases from 5% to 11% of the 31 P signal; the change in intensities of the 76 and 63 ppm resonances leads to a shift in the composite peak maximum from 74 to 70 ppm due to increasing intensity from the more shielded resonance. Similar changes in the 31 P NMR spectra have been observed for glasses with ®xed P2 S5 content (Fig. 4), although there are dierences in the peak shifts on substituting As2 S3 for GeS2 . The binary GeS2 ±P2 S5 glass contains three peaks at 122, 73 and 45 ppm with a small feature at 7 ppm due to water contamination. The shielding of these P sites is altered by the addition of As2 S3 . The 122 ppm resonance moves up®eld to 115 ppm and a peak at 76 ppm begins to dominate the spectra. 4. Discussion
Fig. 4. 31 P MAS NMR spectra of (GeS2 )90ÿy (As2 S3 )y (P2 S5 )10 glasses: (a) y 40, (b) y 35, (c) y 30, (d) y 24, (e) y 20, (f) y 7.5 and (g) y 0 mol% As2 S3 . The chemical shifts are relative to an external solution of 85% H3 PO4 . The small feature at 7 ppm in (f) and (g) is due to an oxide or hydroxyl impurity.
A fundamental issue with thiophosphate glasses concerns the speciation and/or oxidation state of P. From the structure of known crystalline P sulphides [9], P is expected to be either 4- or 3coordinated by S and/or P. When coordinated solely by S, these species constitute tetrahedral S¸PS3=2 and trigonal PS3=2 groups and can be regarded as corresponding to P5 or P3 , respectively. The observed compositional dependence of physical properties of GeAsP sulphide glasses described above can be used, in conjunction with the measured 31 P MAS NMR spectra, to make inferences on the structural role of P in these vitreous materials. The measured reduction in nD (Fig. 1) of these glasses as P2 S5 replaces As2 S3 can be directly ascribed to the smaller polarizability of either P5 or P3 relative to As3 . However, we suggest that the fact that nD also decreases, albeit slightly, as P2 S5 replaces GeS2 (Fig. 1) indicates that a highly polarizable lone electron pair is not localized on P and, therefore, that the dominant P species is the tetrahedral S¸PS3=2 group associated with P5 .
B.G. Aitken, R.E. Youngman / Journal of Non-Crystalline Solids 263&264 (2000) 117±122
This interpretation is consistent with the compositional evolution of kvis , which depends only on the As concentration of these glasses. Were a lone pair associated with P, i.e., P3 , we expect kvis to redshift as P2 S5 replaces GeS2 , which is not observed. The IR absorption data also corroborate this hypothesis. While the lower frequency bands have not been assigned, the 1370 cmÿ1 band is reasonably attributed to the ®rst overtone of the P¸S bond stretching, as Raman spectroscopy of P±S and As±P±S glasses shows that the stretching frequency of this bond lies in the range of 650±690 cmÿ1 [6]. Although the 1370 cmÿ1 absorption band, which becomes more intense with increasing P content, clearly indicates the presence of tetrahedral S¸PS3=2 groups, it does not rule out the simultaneous occurrence of trigonal PS3=2 groups. The decrease in viscosity and corresponding increase in thermal expansion on substituting P for Ge in these glasses can be rationalized on the basis of decreased connectivity of the network structure on replacing GeS4 tetrahedra with either tetrahedral S¸PS3=2 or trigonal PS3=2 groups. Molar volume data, however, are more suggestive of the P±S speciation. On comparing data for these GeS2 ±P2 S5 glass samples to that of stoichiometric GeAs sulphide glasses [10], it is found that Vmol decreases more rapidly as GeS2 is replaced by As2 S3 than when replaced by P2 S5 (Fig. 2). Neutron scattering studies of a 5GeS2 á As2 S3 glass have shown that As occurs as trigonal AsS3 groups [11]. If the dominant P species in stoichiometric Ge(As)P sulphide glasses were analogous PS3=2 groups, we expect, on the basis of the smaller size of P relative to As, GeS2 ±As2 S3 glasses to have a larger Vmol than the corresponding GeS2 ±P2 S5 glass at any given Ge concentration. We suggest that the contrary observation indicates that the dominant structural motif for P in these stoichiometric glass samples is the tetrahedral S¸PS3=2 group. Overall, the measured glass properties of these samples show an approximately linear compositional dependence. The R factors of linear regression ®ts to the data are 97.3%, 93.9%, 96.5% and 99.4% for nD , kvis , Ts and q, respectively. Linear ®ts are somewhat poorer for a and Vmol , being 87.8% and 89.8%, respectively. These results are
121
consistent with a structural model in which the P speciation is constant regardless of composition. This invariance and the fact that the presence of P¸S bonds is demonstrated by the IR absorption spectra constitutes further evidence that P is chie¯y tetrahedrally coordinated in these stoichiometric GeAsP sulphide glasses. This rationalization of the glass properties with structure is also supported by examination of the P environments as deduced from MAS NMR. Although the correlation of P chemical shifts with speciation is not well established for chalcogenide glasses [12±16], the physical properties of these glasses are correlated with the changes in P speciation observed by NMR. Assuming that all P are in S¸PS3=2 groups, the peaks in Figs. 3 and 4 must be due to dierences in next-nearest neighbor con®gurations. This assumption is consistent with the fact that (1) these glasses contain stoichiometric quantities of S and (2) NMR studies of the P±S system showed the presence of such tetrahedral groups at P levels 6 25% [13]. The peak at 116 ppm has the same chemical shift as that observed for P±S glasses [13,17]. Although the distinction between trigonal PS3=2 and tetrahedral S¸PS3=2 cannot be unambiguously determined from isotropic chemical shifts, it is consistent with previous interpretations of the P speciation of P±S glasses [13,17] to assign the down®eld resonance in these spectra to S¸PS3=2 groups with P as next-nearest neighbors. In addition, the population of this environment increases with increasing P2 S5 content, from 5% to 11% of the total P (Fig. 3). The other peaks are assumed to correspond also to S¸PS3=2 tetrahedra, but with varying numbers of Ge and As next-nearest neighbors. The spectrum of the As-free glass (Fig. 4(g)) is dominated by a peak at 45 ppm. This resonance is more shielded than others in these data and, due to the composition, is assigned to S¸PS3=2 units with all (or mostly) Ge next-nearest neighbors. As As2 S3 replaces GeS2 , the formation of peaks between 70 and 80 ppm may correspond to the substitution of As in the next-nearest neighbor coordination shell. As should decrease the shielding, similar to that of P next-nearest neighbors (115 ppm peak). We suggest that these data indicate a microstructure with minor clustering of the P groups, but with a broad
122
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distribution of As and Ge next-nearest neighbors for most of the S¸PS3=2 species. However, the MAS NMR data do not preclude the presence of PS3=2 species due to the substantial overlap of the resonances of trigonal and tetrahedral groups [13].
Acknowledgements
5. Conclusions
References
The slight decrease in nD and Vmol as P2 S5 replaces GeS2 and the concomitant constancy of kvis indicate that the dominant P species in stoichiometric GeAsP sulphide glasses is the tetrahedral S¸PS3=2 group. This interpretation is supported by the presence of an IR absorption band at 1370 cmÿ1 associated with P¸S whose intensity increases monotonically with increasing P2 S5 concentration. The linear compositional dependence of all measured properties further indicates that P coordination is independent of composition. 31 P MAS NMR spectroscopy of these glasses corroborates this interpretation of P speciation. 31 P spectra have resonances that can be assigned solely to tetrahedral S¸PS3=2 groups. The dierent chemical shifts of these resonances are attributed to changes in the identity of next-nearest neighbors: resonances at 120 and 45 ppm are due to tetrahedral P with dominantly P or Ge nextnearest neighbors, respectively. Glasses containing P 20% As2 S3 , however, have a resonance near 75 ppm associated with tetrahedral S¸PS3=2 groups with As as the principal next-nearest neighbor. The tendency of P to associate more strongly with As than with either Ge or P in these glasses may be due to the relative stability of compound-forming As2 P2 S8 structural entities [6,7].
The authors thank M.L. Powley, D.H. Crooker and L.K. Cornelius for their technical assistance in the synthesis of the glass samples used in this study.
[1] B.G. Aitken, M.A. Newhouse, US Pat. 5,389,584, 1995. [2] B.G. Aitken, D.H Crooker, M.L. Powley, US Pat. Appl. 60/064,273, 1997. [3] R.L. Myuller, G.M. Orlova, V.N. Timofeeva, G.I. Ternova, in: Z.U. Borisova (Ed.), Solid State Chemistry, Consultants Bureau, New York, 1996, p. 232. [4] J.A. Savage, S. Nielsen, Infrared Phys. 5 (1965) 195. [5] A.R. Hilton, C.E. Jones, M. Brau, Infrared Phys. 4 (1964) 213. [6] L. Koudelka, M. Pisarcik, L.N. Blinov, M.S. Gutenev, J. Non-Cryst. Solids 134 (1991) 86. [7] A. Tverjanovich, M. Krueger, M. Soltwisch, D. Quitmann, J. Non-Cryst. Solids 130 (1991) 236. [8] E.H. Fontana, Am. Ceram. Soc. Bull. 49 (1970) 594. [9] A. Vos, R. Olthof, F. van Bolhuis, R. Botterweg, Acta Crystallogr. 19 (1965) 864. [10] B.G. Aitken, C.W. Ponader, J. Non-Cryst. Solids 256&257 (1999) 143. [11] A.C. Hannon, B.G. Aitken, J. Non-Cryst. Solids 256&257 (1999) 73. [12] H. Eckert, C.S. Liang, G.D. Stucky, J. Phys. Chem. 93 (1989) 452. [13] M. Tullius, D. Lathrop, H. Eckert, J. Phys. Chem. 94 (1990) 2145. [14] H. Eckert, Z. Zhang, J.H. Kennedy, Chem. Mater. 2 (1990) 273. [15] Z. Zhang, J.H. Kennedy, H. Eckert, J. Am. Chem. Soc. 114 (1992) 5775. [16] J. Schmedt auf der Gunne, H. Eckert, Chem. Eur. J. 4 (1998) 1762. [17] C.M. Lyda, J.M. Leone, M.A. Bankert, Y. Xia, H. Eckert, Chem. Mater. 6 (1994) 1934.
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GeAs thiophosphate glasses: properties and NMR spectroscopy B.G. Aitken *, R.E. Youngman SP-FR-05, Corning Incorporated, Corning, NY 14831, USA
Abstract Stoichiometric GeAsP sulphide glasses containing 50±90 mol% GeS2 and 4±25 mol% P2 S5 have been investigated to determine the compositional and structural dependence of physical properties. The latter vary monotonically with increasing P2 S5 concentration: at constant GeS2 content, the thermal expansion coecient increases, whereas refractive index, isokom temperatures and density decrease as P2 S5 replaces As2 S3 . The introduction of P is marked by absorption bands in the 900±1400 cmÿ1 range that are absent in P-free glasses. The compositional dependence of optical properties and molar volume indicates that the dominant P species in these glasses is the tetrahedral S¸PS3=2 group. This interpretation of the local P environment is con®rmed by 31 P MAS NMR spectral data, which further indicate that the latter species tend to have AsS3 groups as next-nearest neighbors. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Rare-earth doped GeAs sulphide glasses are of interest for the fabrication of a variety of photonic devices, including 1.3 lm optical ampli®ers and mid-IR lasers [1]. Most of these devices require the active glass to be made in the form of an optical ®ber. The latter in turn requires that the core of such a ®ber be clad by a chemically and physically compatible glass with smaller refractive index. P additions to GeAs sulphide glasses have been found to have the desired eect of decreasing the refractive index [2]. Previous work has demonstrated glass formation in the limiting ternary Ge± As±S, Ge±P±S and As±P±S systems [3±7]. This study was undertaken to determine the extent of
* Corresponding author. Tel.: +1-607 974 3111; fax: +1-607 974 3675. E-mail address: [email protected] (B.G. Aitken).
glass formation in the broader quaternary system, as well as the compositional and/or structural dependence of physical properties relevant to the ultimate goal of ®ber fabrication. 2. Experimental GeAsPS glasses were synthesized from 36 g mixtures of elements with P 99.9995% purity (metals basis) that were loaded into 10 mm ID fused silica ampoules under N2 . Prior to batching, the ampoules were etched in 5% HF:5% HNO3 , rinsed in deionized water and then dried at about 1000°C. The ®lled ampoules were evacuated to 10ÿ6 Torr, ¯ame sealed and then heated to 800± 875°C for at least 24 h in a rocking furnace. Cylindrical 15 cm long glass rods were formed by quenching the hot ampoules in water and subsequently annealing near the glass transition temperature (Tg ).
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 6 2 7 - 4
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Density was measured in water to 0.001 g/cm3 using the Archimedes method. Tg was determined by dierential scanning calorimetry at a heating rate of 10°C/min with a precision of 2°C. The viscosity in the 105 ±109 P range was determined by the parallel plate technique [8] using fused silica plates with an experimental error of 2°C. Thermal expansion was measured from room temperature to 200°C to 0.1 ppm/°C by dilatometry using Al2 O3 as a reference. Refractive index was measured to 0.01 by the apparent depth method at 589 nm with 5 mm thick samples. Optical absorption was measured using a spectrophotometer (Cary 5G for visible/near IR and Mattson Cygnus for IR). 31 P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were collected using a 5 mm MAS NMR probe (Chemagnetics) and In®nity console, in conjunction with a 4.7 T superconducting magnet (80.4 MHz resonance frequency). Samples were powdered and packed into ZrO2 rotors. The NMR data were collected at a spinning rate of 12 kHz with single pulse excitation (pulse width 3.3 ls) and recycle delays of 600±900 s. The chemical shifts were referenced to an external solution of 85% H3 PO4 .
3. Results Stoichiometric GeAsPS glasses, i.e., glasses whose chemical composition lies in the GeS2 ± As2 S3 ±P2 S5 plane, can be formed at modest quenching rates from all compositions save for small regions in the immediate vicinity of GeS2 and P2 S5 . The glass samples investigated in this study are all relatively Ge-rich, containing P 50% GeS2 and their physical properties vary monotonically with composition (Table 1). Within the studied range, the refractive index (nD ) varies from 2.18 to 2.48. Upon substituting P2 S5 for As2 S3 , nD decreases by about 0.006 per mol% P2 S5 , but shows no decrease within errors of measurement on substituting P2 S5 for GeS2 (Fig. 1). The glass samples range in colour from yellow to reddish orange, corresponding to a shift of the visible absorption edge (kvis wavelength where the transmission of a 2 mm thick sample is 50% of that at 1.0 lm) from 500 to 560 nm. As with nD , kvis is a function solely of As content, shifting to shorter wavelength as P2 S5 replaces As2 S3 at a constant GeS2 concentration, but remaining unchanged when P2 S5 is substituted for GeS2 . Whereas P-free
Table 1 Physical properties of stoichiometric GeAsP sulphide glasses % GeS2 (mol%)
% P2 S5 (mol%)
nD
kvis (nm)
Tg (°C)
Ts (°C)
a (ppm/°C)
q (g/cm3 )
Vmol (cm3 /mol)
90 83.3 83.3 83.3 83.3 83.3 78.3 72.7 70 66.7 66.7 66.7 66.7 66.7 60 55 50 50 50 50 50
10 4.2 8.3 10 12.5 16.7 4.3 9.1 10 5 10 14.3 20 33.3 10 10 5 10 15 20 25
2.23 2.29 2.26
501 512 508 506 508 506 520 526 523 535 527 528 524
353 331 337
485 466 458
15.8 15.6 16.3
315 300 317 280 270 278 265 282 257 218 253 252 248 239 228 225 218
433 433 421 395 372 373 370 360 337 350
17.2 18.5 16.5 18.8 18.3 17.8 18.6 19.6 20.9 27.7 19.2
331 324 311 303 293
21.3 21.4 22.8 24.0
2.641 2.801 2.722 2.688 2.636 2.558 2.823 2.767 2.767 2.874 2.783 2.701 2.610 2.373 2.814 2.841 2.948 2.855 2.781 2.702 2.619
16.18 16.09 16.06 16.06 16.09 16.09 16.04 15.93 15.90 15.89 15.87 15.90 15.87 16.07 15.82 15.76 15.73 15.76 15.71 15.70 15.74
2.24 2.22 2.31 2.32 2.24 2.38 2.33 2.32 2.29 2.20 2.37 2.41 2.41 2.40 2.35 2.33 2.31
536 541 554 547 538 533 527
B.G. Aitken, R.E. Youngman / Journal of Non-Crystalline Solids 263&264 (2000) 117±122
Fig. 1. Variation of nD of stoichiometric GeAsP sulphide glasses (1) on substituting P2 S5 for As2 S3 at constant GeS2 concentrations of 83, 67 and 50 mol% and (2) on substituting P2 S5 for GeS2 at 0 and 18 mol% As2 S3 . The lines are linear ®ts to the data.
GeAs sulphide glasses have an IR cuto near 850 cmÿ1 , several absorption bands at about 920, 1100, 1190 and 1370 cmÿ1 , whose intensity increases with increasing P2 S5 concentration, are present in the IR spectrum of P-containing glasses. The samples are all low temperature glasses, with Tg ranging from 200°C to 350°C and the softening point (Ts the temperature at which the glass viscosity equals 107:6 P) varying from 290°C to 480°C. Both Tg and Ts decrease with increasing P content. The change in Ts is largest for the case where P2 S5 replaces GeS2 , corresponding to a decrease of about 7.5°C per mol% P2 S5 . The viscosity decrease reduces to about 2.5°C per mol% P2 S5 upon replacing As2 S3 with P2 S5 . The thermal expansion coecient (a) of the studied glasses ranges from 15 to 28 ppm/°C, with its compositional dependence being the inverse of that of the viscosity: a increases with increasing P concentration, with the rate of increase being greater as P replaces Ge. The density of these glasses varies from 2.3 to 3.0 gm/cc and, as expected from the smaller atomic mass of P, decreases with increasing P content. The molar volume (Vmol , where the latter is calculated on a constant atom basis using molecular
119
formulae of the form Gex Asy Pz S1ÿxÿy ÿz ) decreases by about 0.01 cm3 /mol% P2 S5 as P2 S5 replaces GeS2 at constant As2 S3 content (Fig. 2). However, its decrease is an order of magnitude less as P2 S5 is substituted for As2 S3 . Finally, the durability of stoichiometric GeAsP sulphide glasses decreases with increasing P content. After exposure to ambient humidity, glasses containing in excess of about 12% P2 S5 develop an opaque yellow hydrated surface layer, which is con®rmed by the appearance of peaks near 0 ppm in the 31 P MAS NMR spectra. Portions of the same P-rich glass samples retain a pristine surface when stored in a desiccator. 31 P NMR spectra for two series of these samples at 50% GeS2 and 10% P2 S5 are provided in Figs. 3 and 4, respectively. The data for the former series are aected by microstructural changes as a function of substituting P2 S5 for As2 S3 . The spectrum of the 5% P2 S5 glass has two features: a smaller symmetric peak at 116 ppm and a more intense and asymmetric peak at about 74 ppm. The latter
Fig. 2. Compositional dependence of Vmol of stoichiometric GeAsP sulphide glasses (1) on substituting P2 S5 for As2 S3 at 83, 67 and 50 mol% GeS2 and (2) on substituting P2 S5 for GeS2 at 0% As2 S3 . For comparison with the latter series, data for stoichiometric GeAs sulphide glasses from Ref. [10] are plotted as a function of As2 S3 concentration. The lines are linear ®ts to the data. Error bars are equal in size to or smaller than the data symbols.
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Fig. 3. 31 P MAS NMR spectra of (GeS2 )50 (As2 S3 )50ÿx (P2 S5 )x glasses: (a) x 5, (b) x 10, (c) x 15, (d) x 20 and (e) x 25 mol% P2 S5 . The data have been referenced to 85% H3 PO4 . Three Gaussian peaks are indicated by the dashed curves in (a). A vertical line has been drawn at 74 ppm as a guide to the eye.
feature can be deconvoluted into two overlapping Gaussian peaks centered at 76 and 63 ppm. The gradual replacement of As2 S3 by P2 S5 does not aect the chemical shifts of these peaks, but does change their relative intensities. The resonance at 116 ppm increases from 5% to 11% of the 31 P signal; the change in intensities of the 76 and 63 ppm resonances leads to a shift in the composite peak maximum from 74 to 70 ppm due to increasing intensity from the more shielded resonance. Similar changes in the 31 P NMR spectra have been observed for glasses with ®xed P2 S5 content (Fig. 4), although there are dierences in the peak shifts on substituting As2 S3 for GeS2 . The binary GeS2 ±P2 S5 glass contains three peaks at 122, 73 and 45 ppm with a small feature at 7 ppm due to water contamination. The shielding of these P sites is altered by the addition of As2 S3 . The 122 ppm resonance moves up®eld to 115 ppm and a peak at 76 ppm begins to dominate the spectra. 4. Discussion
Fig. 4. 31 P MAS NMR spectra of (GeS2 )90ÿy (As2 S3 )y (P2 S5 )10 glasses: (a) y 40, (b) y 35, (c) y 30, (d) y 24, (e) y 20, (f) y 7.5 and (g) y 0 mol% As2 S3 . The chemical shifts are relative to an external solution of 85% H3 PO4 . The small feature at 7 ppm in (f) and (g) is due to an oxide or hydroxyl impurity.
A fundamental issue with thiophosphate glasses concerns the speciation and/or oxidation state of P. From the structure of known crystalline P sulphides [9], P is expected to be either 4- or 3coordinated by S and/or P. When coordinated solely by S, these species constitute tetrahedral S¸PS3=2 and trigonal PS3=2 groups and can be regarded as corresponding to P5 or P3 , respectively. The observed compositional dependence of physical properties of GeAsP sulphide glasses described above can be used, in conjunction with the measured 31 P MAS NMR spectra, to make inferences on the structural role of P in these vitreous materials. The measured reduction in nD (Fig. 1) of these glasses as P2 S5 replaces As2 S3 can be directly ascribed to the smaller polarizability of either P5 or P3 relative to As3 . However, we suggest that the fact that nD also decreases, albeit slightly, as P2 S5 replaces GeS2 (Fig. 1) indicates that a highly polarizable lone electron pair is not localized on P and, therefore, that the dominant P species is the tetrahedral S¸PS3=2 group associated with P5 .
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This interpretation is consistent with the compositional evolution of kvis , which depends only on the As concentration of these glasses. Were a lone pair associated with P, i.e., P3 , we expect kvis to redshift as P2 S5 replaces GeS2 , which is not observed. The IR absorption data also corroborate this hypothesis. While the lower frequency bands have not been assigned, the 1370 cmÿ1 band is reasonably attributed to the ®rst overtone of the P¸S bond stretching, as Raman spectroscopy of P±S and As±P±S glasses shows that the stretching frequency of this bond lies in the range of 650±690 cmÿ1 [6]. Although the 1370 cmÿ1 absorption band, which becomes more intense with increasing P content, clearly indicates the presence of tetrahedral S¸PS3=2 groups, it does not rule out the simultaneous occurrence of trigonal PS3=2 groups. The decrease in viscosity and corresponding increase in thermal expansion on substituting P for Ge in these glasses can be rationalized on the basis of decreased connectivity of the network structure on replacing GeS4 tetrahedra with either tetrahedral S¸PS3=2 or trigonal PS3=2 groups. Molar volume data, however, are more suggestive of the P±S speciation. On comparing data for these GeS2 ±P2 S5 glass samples to that of stoichiometric GeAs sulphide glasses [10], it is found that Vmol decreases more rapidly as GeS2 is replaced by As2 S3 than when replaced by P2 S5 (Fig. 2). Neutron scattering studies of a 5GeS2 á As2 S3 glass have shown that As occurs as trigonal AsS3 groups [11]. If the dominant P species in stoichiometric Ge(As)P sulphide glasses were analogous PS3=2 groups, we expect, on the basis of the smaller size of P relative to As, GeS2 ±As2 S3 glasses to have a larger Vmol than the corresponding GeS2 ±P2 S5 glass at any given Ge concentration. We suggest that the contrary observation indicates that the dominant structural motif for P in these stoichiometric glass samples is the tetrahedral S¸PS3=2 group. Overall, the measured glass properties of these samples show an approximately linear compositional dependence. The R factors of linear regression ®ts to the data are 97.3%, 93.9%, 96.5% and 99.4% for nD , kvis , Ts and q, respectively. Linear ®ts are somewhat poorer for a and Vmol , being 87.8% and 89.8%, respectively. These results are
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consistent with a structural model in which the P speciation is constant regardless of composition. This invariance and the fact that the presence of P¸S bonds is demonstrated by the IR absorption spectra constitutes further evidence that P is chie¯y tetrahedrally coordinated in these stoichiometric GeAsP sulphide glasses. This rationalization of the glass properties with structure is also supported by examination of the P environments as deduced from MAS NMR. Although the correlation of P chemical shifts with speciation is not well established for chalcogenide glasses [12±16], the physical properties of these glasses are correlated with the changes in P speciation observed by NMR. Assuming that all P are in S¸PS3=2 groups, the peaks in Figs. 3 and 4 must be due to dierences in next-nearest neighbor con®gurations. This assumption is consistent with the fact that (1) these glasses contain stoichiometric quantities of S and (2) NMR studies of the P±S system showed the presence of such tetrahedral groups at P levels 6 25% [13]. The peak at 116 ppm has the same chemical shift as that observed for P±S glasses [13,17]. Although the distinction between trigonal PS3=2 and tetrahedral S¸PS3=2 cannot be unambiguously determined from isotropic chemical shifts, it is consistent with previous interpretations of the P speciation of P±S glasses [13,17] to assign the down®eld resonance in these spectra to S¸PS3=2 groups with P as next-nearest neighbors. In addition, the population of this environment increases with increasing P2 S5 content, from 5% to 11% of the total P (Fig. 3). The other peaks are assumed to correspond also to S¸PS3=2 tetrahedra, but with varying numbers of Ge and As next-nearest neighbors. The spectrum of the As-free glass (Fig. 4(g)) is dominated by a peak at 45 ppm. This resonance is more shielded than others in these data and, due to the composition, is assigned to S¸PS3=2 units with all (or mostly) Ge next-nearest neighbors. As As2 S3 replaces GeS2 , the formation of peaks between 70 and 80 ppm may correspond to the substitution of As in the next-nearest neighbor coordination shell. As should decrease the shielding, similar to that of P next-nearest neighbors (115 ppm peak). We suggest that these data indicate a microstructure with minor clustering of the P groups, but with a broad
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distribution of As and Ge next-nearest neighbors for most of the S¸PS3=2 species. However, the MAS NMR data do not preclude the presence of PS3=2 species due to the substantial overlap of the resonances of trigonal and tetrahedral groups [13].
Acknowledgements
5. Conclusions
References
The slight decrease in nD and Vmol as P2 S5 replaces GeS2 and the concomitant constancy of kvis indicate that the dominant P species in stoichiometric GeAsP sulphide glasses is the tetrahedral S¸PS3=2 group. This interpretation is supported by the presence of an IR absorption band at 1370 cmÿ1 associated with P¸S whose intensity increases monotonically with increasing P2 S5 concentration. The linear compositional dependence of all measured properties further indicates that P coordination is independent of composition. 31 P MAS NMR spectroscopy of these glasses corroborates this interpretation of P speciation. 31 P spectra have resonances that can be assigned solely to tetrahedral S¸PS3=2 groups. The dierent chemical shifts of these resonances are attributed to changes in the identity of next-nearest neighbors: resonances at 120 and 45 ppm are due to tetrahedral P with dominantly P or Ge nextnearest neighbors, respectively. Glasses containing P 20% As2 S3 , however, have a resonance near 75 ppm associated with tetrahedral S¸PS3=2 groups with As as the principal next-nearest neighbor. The tendency of P to associate more strongly with As than with either Ge or P in these glasses may be due to the relative stability of compound-forming As2 P2 S8 structural entities [6,7].
The authors thank M.L. Powley, D.H. Crooker and L.K. Cornelius for their technical assistance in the synthesis of the glass samples used in this study.
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