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Structure and properties of lanthanum–aluminum–phosphate glasses
Journal of Non-Crystalline Solids 283 (2001) 211±219
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Structure and properties of lanthanum±aluminum±phosphate glasses M. Karabulut a,b, E. Metwalli a,c, R.K. Brow a,* a
Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65401, USA b Department of Physics, University of Kafkas, Kars, Turkey c National Research Center, Glass Research Department, Dokki, Cairo, Egypt Received 14 August 2000
Abstract Glass-forming characteristics, properties and structural features of glasses in the La2 O3 ±Al2 O3 ±P2 O5 system have been investigated. The glass-forming region is small compared to the Na2 O±Al2 O3 ±P2 O5 system. Glass transition temperature increases and thermal expansion coecient, refractive index, and density all decrease as the alumina content of the glass is increased. Infrared (IR) spectroscopy indicates that the glass network is dominated by bridging Ptetrahedra with terminal tetrahedra present in glasses with O/P ratios > 3. 27 Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy indicates that Al ions are incorporated in these glasses in 4-, 5-, and 6-coordinated sites. The average Al-coordination number (CN) increases with an increase in the Al/La ratio and decreases with an increase in the O/P ratio. Both trends can be explained by the avoidance of Al±O±Al bonds in the glass structure. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Phosphate glasses have been developed for a variety of applications. Rare-earth containing metaphosphate glasses possess high stimulated emission cross-sections and low thermo-optical coecients and are the primary host materials for high-power laser applications [1]. The high thermal expansion coecient and low glass transition temperature of alkali aluminophosphate glasses make them useful for hermetic sealing technology [2,3]. The chemical durability of iron phosphate glasses is better than that of the borosilicate glasses and up to 50 wt% of certain nuclear waste can be
*
Corresponding author. Fax: +1-573 341 6934. E-mail address: [email protected] (R.K. Brow).
vitri®ed in these glasses, as a result they are potential candidates as a host matrix to vitrify certain high-level nuclear wastes [4]. These applications often require compositions that possess outstanding chemical durability. Introducing Al2 O3 into sodium phosphate glass network increases the cross links between PO4 tetrahedra in the glass which results in an increase in the aqueous durability and glass transition temperature, and a decrease in thermal expansion coecient [5]. The glass-forming characteristics, and structure±property relations in the ternary alkali M2 O±Al2 O3 ±P2 O5 (M Li, Na, K) and alkaline earth MO±Al2 O3 ±P2 O5 (M Mg, Ca, Ba) phosphate systems have been investigated previously [5,6]. Less is known about the nature of aluminophosphate glasses modi®ed by ternary oxides. In the present study, we have studied the
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 4 2 0 - 3
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M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
glass forming characteristics, properties and structural features of glasses in the La2 O3 ± Al2 O3 ±P2 O5 system. 2. Experimental details 2.1. Glass preparation and characterization Glasses were synthesized by melting mixtures of reagent grade chemicals, LaPO4 or La2 O3 ; Al PO3 3 or Al2 O3 , and NH4 H2 PO4 in high purity alumina crucibles in air for approximately 1 h at temperatures between 1250°C and 1500°C depending on the composition. Each batch was calcined at 500°C for 4 h, then at 900°C for 1 h, before reaching the melting temperature. The top of the crucible was covered during melting. Melts were quenched in stainless steel moulds and annealed at 600°C for 2 h. Glass compositions are divided into three groups: Series I include metaphosphate glasses (O/ P 3) with the general composition of 25 x La2 O3 ±xAl2 O3 ±75P2 O5 ; Series II glasses include glasses with general composition of 30 x La2 O3 ±xAl2 O3 ±70P2 O5 . Additional compositions were prepared to determine the glass-forming range. The compositions of selected glasses were checked by the energy dispersive analysis of X-rays (EDAX). At least ®ve spots were examined on each sample and the average composition is reported. These analyses were reproducible to 5%. 2.2. Property measurements The density (q) of the glasses was measured by Archimedes' method, using kerosene as the immersion ¯uid. Measurements were made in duplicate for each glass and the averages are reported. The refractive index (n) of glasses was measured using the Becke line method and the uncertainty in the reported values is 0:002. Glass transition temperature (Tg ) was measured by dierential thermal analysis (DTA) in platinum pans under nitrogen gas. Each sample was heated to above the anticipated Tg , then cooled at 10°C= min to 300°C to eliminate the eect of ®ctive temperature on Tg , before reheating at 10°C/min to record
Tg . These measurements are reproducible within 3°C. The average thermal expansion coecient (a) between 200°C and 500°C for each glass was determined by dilatometry with a heating rate of 5°C/ min. The estimated error in a is 3%. The chemical durability of the bulk glasses with approximate size of 1 1 1 cm3 was evaluated by weight loss measurements from glasses exposed to deionized water at 70°C for eight days. Glasses were polished to 600 grit ®nish with SiC paper, cleaned with acetone and suspended in polypropylene containers containing 100 ml of deionized water at 70°C. Measurements were conducted in duplicate for each glass and the average dissolution rate (DR), normalized to the glass surface area and the corrosion time, was calculated from the weight loss at the conclusion of the test. 2.3. Structural investigation 27
Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were collected at 94.67 MHz at a MAS frequency of 10 kHz. 1.2 ls pulses and 300 ms recycle times were used. 1 M Al NO3 3 was used as a reference for the chemical shifts. The peak maxima are reported as the peak positions. Relative site concentrations of dierent aluminum species in glasses have been taken to be the relative areas of the Gaussian curves used to deconvolute the 27 Al MAS NMR spectra. The infrared (IR) spectrum for each glass was collected between 400 and 2000 cm 1 using a Perkin±Elmer 1760X FT-IR spectrometer. Samples were prepared as pellets by pressing a mixture of glass powder and anhydrous KBr powder. The spectrometer was purged with dry nitrogen and then 20 scans were collected for each sample and the average spectrum was recorded. A pure KBr spectrum was subtracted from each glass spectrum to correct the background. 3. Results 3.1. Glass forming and properties The batch compositions of the glasses are given in Table 1. EDAX analyses of selected glasses are
Glass
Composition (mol%)
Tg °C
a 1=°C
q g=cm3
n
Log DR
75 (73.7) 75 75 (75) 75
618 631 643 685
nm 72 66 66
3.31 3.01 2.92 2.77
1.568 1.572 1.554 1.538
20 (22) 25 (27)
75 (73) 75 (73)
754 814
61 nm
2.67 2.58
1.526 1.526
nm )7.5 )7.3 No detectable wt loss )7.2 nm
0 10 (14) 15 (18.1) 20 (26.4)
70 70 70 70
(69) (66) (66.7) (64.6)
632 645 644 654
nm 65 49 46
3.42 3.12 2.95 2.79
1.604 1.576 1.552 1.530
5 (5) 0
25 (29.8) 30 (32)
70 (65.2) 70 (68)
669 754
) nm
2.65 2.52
1.518 1.512
nm Cracked )6.7 No detectable wt loss nm nm
24.375 23.75 17.5
2.5 5 15
73.125 71.25 67.5
638 640 643
72 74 50
3.27 3.25 3.01
1.576 1.574 1.562
)7 )6.9 )7.2
La2 O3
Al2 O3
P2 O5
25La75P SI-1 SI-2 SI-3
25 (26.3) 20 15 (11.5) 10
0 5 10 (13.5) 15
SI-4 25Al75P
5 (5) 0
30La70P SII-l SII-2 SII-3
30 20 15 10
SII-4 30Al70P 24.375La2.5Al 23.75La5Al 17.5La15Al a
(31) (20) (15.2) (9)
The compositions of selected glasses were analyzed by EDAX and the results are given in parentheses. The analyzed compositions and selected properties of binary La2 O3 ±P2 O5 and Al2 O3 ±P2 O5 glasses prepared as the end members of Series I and II glasses are also included for comparison.
M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
Table 1 Composition and selected properties of the LAP glassesa
213
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M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
given in parentheses. In general, some P2 O5 (< 8% relative) was lost during sample preparation. Glasses batched with 80 mol% P2 O5 had ®nal compositions similar to the metaphosphate glasses in Series I and so are not considered here. (Ultraphosphate compositions likely need to be prepared in sealed ampoules to avoid signi®cant P2 O5 volatilization [7].) The glass-forming region is given as a solid line in Fig. 1; open circles indicate glass-forming compositions and the compositions that crystallized upon quenching are represented as ®lled circles. LaPO4 and AlPO4 were present in crystallized samples of these latter La2 O3 -rich and Al2 O3 -rich compositions, respectively. Glass formation is expected to extend to the P2 O5 -corner of Fig. 1. The properties of the LAP glasses are given in Table 1. The replacement of La2 O3 and P2 O5 with Al2 O3 increases Tg and decreases a for almost all the glasses studied. The changes in Tg with Al2 O3 for Series I and II glasses are shown in Fig. 2. The increase in Tg is greater for Series I glasses than Series II glasses. The density and refractive index of the LAP glasses are given in Table 1 and shown in Fig. 3. Both density and refractive index decrease monotonically with the addition of alumina in both series. The DRs calculated from the weight loss
Volatile compositions
Fig. 2. Compositional dependence of glass transition temperature as a function of the batched Al2 O3 content for Series I and II glasses. The glass transition temperatures for the binary La2 O3 ±P2 O5 and Al2 O3 ±P2 O5 glasses (labeled `b') prepared as the end members of Series I and II glasses are included for comparison. The lines are drawn as guides for the eye.
experiments in distilled water at 70°C for eight days are given in the last column of Table 1. The DRs are in the 10 7 ±10 8 g=cm2 = min range for all the glasses studied. For SI-3 (15 mol% Al2 O3 ) and SII-3 (20 mol% Al2 O3 ) glasses, no weight loss was detectable. SII-1 glass containing 10 mol% Al2 O3 cracked during the weight loss experiment. For comparison, commercial soda±lime silicate glasses have dissolution rates in the 10 8 g=cm2 = min range in distilled water at 70°C. 3.2. Structural investigations
Fig. 1. Glass-forming region in La2 O3 ±Al2 O3 ±P2 O5 (batch composition) ternary system. Solid circles correspond to compositions that were partially or fully crystallized. The binary La2 O3 ±P2 O5 and Al2 O3 ±P2 O5 glasses (analyzed composition) prepared as the end members for Series I and II glasses are also included.
3.2.1. 27 Al MAS NMR spectra The 27 Al MAS NMR spectra for the glasses in Series I and II are given in Figs. 4(a) and (b). The spectra yield three peaks, near +33, +4, and )20 ppm. Similar peaks were observed in the 27 Al MAS NMR spectra from sodium aluminophosphate
M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
Fig. 3. Compositional dependence of density and refractive index plotted as a function of the batched Al2 O3 content for Series I and II glasses. The density and refractive index for the binary La2 O3 ±P2 O5 and Al2 O3 ±P2 O5 glasses (labeled `b') prepared as the end members of Series I and II glasses are included for comparison. Lines are drawn as guides for the eye.
(a)
215
glasses and they were assigned to Al OP4 ; Al OP5 , and Al OP6 , sites, respectively [5]. We use the same assignments for peaks observed in the LAP spectra. The positions and relative areas of the three peaks are given in Table 2. (Each peak position in the spectra from the LAP glasses is shifted 5±8 ppm up®eld from those reported for Na-aluminophosphate glasses [5], re¯ecting the greater ®eld strength of La3 on the 27 Al NMR spectra.) In Series I, the spectra are dominated by peaks representing Al(6) sites and the intensity of the peak corresponding to these sites increases with the increasing alumina content (Table 2 and Fig. 4(a)). In Series II, the spectra are dominated by peaks representing the Al(4) sites and the intensity of the peak corresponding to these sites decreases as alumina content in glass increases (Table 2, Fig. 4(b)). It has been shown that the quadrupolar eects do not cause signi®cant changes in the relative peak intensity of the 27 Al MAS NMR spectra of sodium aluminophosphate glasses [5]. Similarly, if we assume that no signal is lost due to the quadrupolar interactions, the relative 27 Al MAS NMR peak areas are proportional to the relative
(b)
Fig. 4. 27 Al MAS NMR spectra from: (a) Series I glasses [ 25 xLa2 O3 ±xAl2 O3 ±75P2 O5 , mol%]; (b) Series II glasses [ 30 xLa2 O3 ±xAl2 O3 ±70P2 O5 , mol%]. The spectra of binary glasses are included (dashed lines) for comparison.
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M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
Table 2 Chemical shifts (ppm) and relative areas (RA) of aluminum sites in LAP glasses obtained from deconvolution of spectraa Glass
Al (6)
Al (5)
Al (4)
27
Al MAS NMR
ppm
RA
ppm
RA
ppm
RA
Average Al CN
SI-1 SI-2 SI-3 SI-4 27Al73P
)20.3 )21.0 )21.3 )21.3 )21.5
0.43 0.47 0.52 0.63 0.61
4.3 2.5 3.0 3.0 2.2
0.32 0.31 0.31 0.26 0.30
33.7 32.7 32.3 32.3 32.3
0.25 0.22 0.17 0.11 0.09
5.18 5.26 5.34 5.51 5.55
SII-l SII-2 SII-3 SII-4 32Al68P
)18.1 )20.0 )20.8 )20.3 )22.0
0.21 0.24 0.27 0.32 0.42
4.8 3.5 3.5 3.0 2.0
0.31 0.33 0.35 0.36 0.36
33.7 33.2 33.2 31.1 32.0
0.48 0.43 0.38 0.32 0.22
4.72 4.81 4.89 5.00 5.25
24.375La2.5Al 23.75La5Al 17.5La15Al
)20.1 )19.3 )19.5
0.38 0.27 0.15
2.1 4.6 4.8
0.31 0.33 0.29
32.0 33.2 33.7
0.31 0.40 0.56
5.07 4.88 4.60
a
The results for two binary Al2 O3 ±P2 O5 glasses prepared as the end members of Series I and II glasses are also listed.
concentrations of the dierent Al sites and so can be used to calculate the average Al-coordination numbers (CN) given in the last column of Table 2. The average Al-CN in Series I glasses ranges from 5.2 to 5.6; for Series II, Al-CN is between 4.7 and 5.0. The eect of composition on Al-CN for both series is shown in Fig. 5. For both series, Al-CN increases with increasing Al2 O3 content. 3.2.2. IR measurements The IR spectra of glasses in Series I and II are shown in Figs. 6(a) and (b). The spectra are very similar to those reported for various metaphosphate glasses [8,9]. In general, there are ®ve major bands observed at around 1267, 1073, 912, 778, and 490 cm 1 . These bands can be assigned to the mas PO2 ; ms PO2 ; mas POP; ms POP, and dPO2 modes of PO3 n chain groups, respectively [9,10]. The intensity of strong bands observed around 1267, 1073 and 912 cm 1 for the glass containing 5 mol% Al2 O3 (Fig. 6(a)) decreases as the Al2 O3 content increases, and their frequencies shift to higher values. Similar behavior is observed for glasses in Series II (Fig. 6(b)). The overall intensity of the mas PO2 and ms PO2 bands is lower and the eect of the addition of alumina is more pronounced in the Series II glasses. The intensity of the band at 1080 cm 1 ms PO2 decreases as the
Al2 O3 content in the glass increases and it disappears totally and a new band at 1154 cm 1 emerges for the Series II glass containing 25 mol%
Fig. 5. Average Al-CN for the glasses in Series I and II determined from 27 Al MAS NMR spectra. The average Al-CN for the binary Al2 O3 ±P2 O5 glasses (labeled `b') prepared as the end members of Series I and II glasses are included for comparison. The lines are drawn as guides for the eye.
M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
(a) Fig. 6. IR spectra from: (a) Series I glasses [ 25 70P2 O5 , mol%].
217
(b) xLa2 O3 ±xAl2 O3 ±75P2 O5 , mol%]; (b) Series II glasses [ 30
Al2 O3 . This new band is assigned to the asymmetric vibrations of mas PO3 units [8±10]. 4. Discussion The addition of alumina to sodium phosphate glass strengthens the network by creating crosslinks between phosphate chains. The resulting increase in structural connectivity increases Tg , decreases thermal expansion and improves aqueous durability [5]. Similar property trends are observed for the LAP system (Table 1, Figs. 2 and 3). The decrease in thermal expansion coecient observed when Al2 O3 replaces La2 O3 is not as pronounced as the increase in Tg . Density and refractive index show similar compositional dependence as both properties decrease almost monotonically with increasing Al2 O3 content. The decreases (Fig. 3) are due to the replacement of more massive La3 ions by Al3 ions. There are no breaks in the properties as the alumina content in the glass is increased. For sodium aluminosilicate (NAS) and sodium aluminophosphate (NAP) glasses, a break in the properties is observed when Al2 O3 is added to the glass. In the NAS system, the break occurs when the [Al]/[Si] ratio in the glass exceeds 1.0, at
xLa2 O3 ±xAl2 O3 ±
which point all non-bridging oxygen (NBO) bonds (Si±O Na ) are converted into Si±O±Al(4) bonds [11]. In the NAP system, property breaks are observed when the [O]/[P] ratio exceeds 3.5, the pyrophosphate stoichiometry, and the dominant Al-CN changes from octahedral to tetrahedral [5]. No such breaks in properties (or Al-CN) are observed for the LAP glasses investigated in this study primarily because aluminophosphate glasses with such high O/P ratios could not be made with La2 O3 . The intensities of the IR bands assigned to the PO2 units decrease with increasing concentrations of Al2 O3 in Series I (Fig. 6(a)); however, there are no signi®cant structural changes to the metaphosphate network and there is no evidence for the presence of signi®cant concentrations of terminal groups (mas PO3 at 1154 cm 1 ). These observations are consistent with the analyzed compositions which yield O/P ratios of 3:0. The glasses in Series II, on the other hand, have O/P ratios of 3.3 and so possess structures with signi®cant concentrations of terminal PO3 groups as indicated by the presence of the IR band at 1154 cm 1 (Fig. 6(b)). The observed increase in the frequencies of the asymmetric and symmetric stretching PO2 bands and the decrease in their intensities with increasing
218
M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
Al2 O3 content, for both Series I and II, may be attributed to greater electronegativity of Al3 (1.5) compared to that of La3 (1.1) [12]. The greater electronegativity makes the P±O bond more ionic by increasing the charge separation between phosphorus and oxygen ions which decreases the intensity of the vibration. Peak intensity is inversely proportional to frequency because the electronegativity, which aects the vibrational force constant, and the size of the cation, which aects the P±O bond angle, have opposite eects on the PO2 frequency [13]. According to the vibrational model of Rouse et al., the ms PO2 shifts to higher frequency while mas PO2 shifts to lower frequencies when larger cations are replaced by smaller cations [14]. However, in the IR spectra shown in Fig. 6, both ms PO2 and mas PO2 bands shift to higher frequencies, indicating that the metal± oxygen force constant has the dominant eect on the structure of these glasses. The structural roles of La3 and Al3 in these glasses will be dierent. This can be better understood by comparing the structures of binary aluminum and lanthanum phosphate crystals and glasses. Crystalline La PO3 3 has a structure based on in®nite chains of corner-sharing PO4 tetrahedra and edge-sharing LaO8 dodecahedra [15]. In contrast, the Al3 ions in crystalline Al PO3 3 do not share common oxygens; every Al(6) is isolated by P±O±Al(6) bonds to the in®nite metaphosphate chains [16]. Neutron and X-ray diraction studies of lanthanum metaphosphate glass show that the La±O-CN is 7 [17]. This exceeds the number of terminal oxygens (6) that are available to coordinate each La3 cation, leading to the clustering of the LaOn polyhedra. In the structures of binary aluminophosphate glasses, on the other hand, tetrahedral Al replaces octahedral Al with increasing Al2 O3 content in order to avoid the formation of Al±O±Al bonds [18]. A recent multidimensional NMR study of aluminum containing phosphate glass-ceramics con®rms this `aluminum avoidance' behavior [19]. The increase in the average Al-CN as Al2 O3 replaces La2 O3 in both Series I and II (Fig. 5) can be explained by considering the respective coordination environments of Al3 and La3 in a phosphate glass. If we assume that the La-CN is
7, as reported for La PO3 3 glass [17], then the La3 ions must use most of the NBOs available in the glass, leaving fewer NBOs to coordinate the Al3 ions; hence, Al-CN is low. As Al2 O3 replaces La2 O3 , greater numbers of NBOs (per trivalent cation) are made available and the average Al-CN increases. The average Al-CN is lower for Series II glasses than Series I glasses (Fig. 5) because the former series has a fewer NBOs per trivalent cation and the lower Al-CN is necessary to avoid Al±O±Al linkages.
5. Summary Glass-forming characteristics, properties and structure of glasses in the La2 O3 ±Al2 O3 ±P2 O5 system have been investigated. Tg increases and thermal expansion coecient, refractive index, and density all decrease as the alumina content increases. IR spectroscopy reveals that the glass networks are dominated by bridging P-tetrahedra that constitute the metaphosphate chains, with some chain-terminating sites in glasses with O/P ratios > 3:0. 27 Al MAS NMR reveals that Al3 ions are incorporated into the phosphate networks as 4-, 5-, and 6-coordinated sites. The average AlCN depends on the Al/La and O/P ratios and changes with composition to avoid the formation of Al±O±Al linkages.
Acknowledgements Acknowledgement is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research. The authors thank Clarissa Vierrether, for providing the EDAX data, and Holly Bentley, for assistance in the preparation and characterization of the LAP glasses. References [1] J.H. Campbell, T.I. Suratwala, J. Non-Cryst. Solids 263&264 (2000) 318. [2] Y.B. Peng, D.E. Day, Glass Technol. 32 (1991) 166.
M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219 [3] R.K. Brow, L. Kovacic, R.E. Loehman, Ceram. Trans. 70 (1996) 177. [4] D.E. Day, Z. Wu, C.S. Ray, P. Hrma, J. Non-Cryst. Solids 241 (1998) 1. [5] R.K. Brow, R.J. Kirkpatrick, G.L. Turner, J. Am. Ceram. Soc. 76 (1993) 919. [6] A. Kishioka, M. Hayashi, M. Kinoshita, Bull. Chem. Soc. Jpn. 49 (1976) 3032. [7] H.D. Park, E.R. Kreidler, J. Am. Ceram. Soc. 67 (1984) 23. [8] G.J. Exarhos, P.J. Miller, W.M. Risen Jr., J. Chem. Phys. 60 (1974) 4145. [9] J.J. Hudgens, S.W. Martin, J. Am. Ceram. Soc. 76 (1993) 1691. [10] S. Bruni, F. Cariati, D. Narducci, Vib. Spectrosc. 7 (1994) 169. [11] W. Tasker, D.R. Uhlmann, P.I.K. Onorato, M.N. Alexander, C.W. Struck, J. Phys. 46 (1985) C8-273.
219
[12] L. Pauling, in: The Nature of the Chemical Bond, 3rd Ed., Cornell University, Ithaca, NY, 1960. [13] B.N. Nelson, G.J. Exarhos, J. Chem. Phys. 71 (1979) 2739. [14] G.B. Rouse Jr., P.J. Miller, W.M. Risen Jr., J. Non-Cryst. Solids 28 (1978) 193. [15] J. Matuszewski, J. Kropiwnicka, T. Znamierowska, J. NonCryst. Solids 290 (1988) 285. [16] H. van der Meer, Acta Crystallogr. B 32 (1976) 2423. [17] U. Hoppe, R. Kranold, D. Stachel, A. Barz, A.C. Hannon, J. Non-Cryst. Solids 232&234 (1998) 44. [18] R.K. Brow, C.A. Click, T.M. Alam, J. Non-Cryst. Solids 274 (2000) 9. [19] P. Hartmann, J. Vogel, U. Friedrich, C. Jager, J. NonCryst. Solids 263&264 (2000) 94.
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Structure and properties of lanthanum±aluminum±phosphate glasses M. Karabulut a,b, E. Metwalli a,c, R.K. Brow a,* a
Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65401, USA b Department of Physics, University of Kafkas, Kars, Turkey c National Research Center, Glass Research Department, Dokki, Cairo, Egypt Received 14 August 2000
Abstract Glass-forming characteristics, properties and structural features of glasses in the La2 O3 ±Al2 O3 ±P2 O5 system have been investigated. The glass-forming region is small compared to the Na2 O±Al2 O3 ±P2 O5 system. Glass transition temperature increases and thermal expansion coecient, refractive index, and density all decrease as the alumina content of the glass is increased. Infrared (IR) spectroscopy indicates that the glass network is dominated by bridging Ptetrahedra with terminal tetrahedra present in glasses with O/P ratios > 3. 27 Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy indicates that Al ions are incorporated in these glasses in 4-, 5-, and 6-coordinated sites. The average Al-coordination number (CN) increases with an increase in the Al/La ratio and decreases with an increase in the O/P ratio. Both trends can be explained by the avoidance of Al±O±Al bonds in the glass structure. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Phosphate glasses have been developed for a variety of applications. Rare-earth containing metaphosphate glasses possess high stimulated emission cross-sections and low thermo-optical coecients and are the primary host materials for high-power laser applications [1]. The high thermal expansion coecient and low glass transition temperature of alkali aluminophosphate glasses make them useful for hermetic sealing technology [2,3]. The chemical durability of iron phosphate glasses is better than that of the borosilicate glasses and up to 50 wt% of certain nuclear waste can be
*
Corresponding author. Fax: +1-573 341 6934. E-mail address: [email protected] (R.K. Brow).
vitri®ed in these glasses, as a result they are potential candidates as a host matrix to vitrify certain high-level nuclear wastes [4]. These applications often require compositions that possess outstanding chemical durability. Introducing Al2 O3 into sodium phosphate glass network increases the cross links between PO4 tetrahedra in the glass which results in an increase in the aqueous durability and glass transition temperature, and a decrease in thermal expansion coecient [5]. The glass-forming characteristics, and structure±property relations in the ternary alkali M2 O±Al2 O3 ±P2 O5 (M Li, Na, K) and alkaline earth MO±Al2 O3 ±P2 O5 (M Mg, Ca, Ba) phosphate systems have been investigated previously [5,6]. Less is known about the nature of aluminophosphate glasses modi®ed by ternary oxides. In the present study, we have studied the
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 4 2 0 - 3
212
M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
glass forming characteristics, properties and structural features of glasses in the La2 O3 ± Al2 O3 ±P2 O5 system. 2. Experimental details 2.1. Glass preparation and characterization Glasses were synthesized by melting mixtures of reagent grade chemicals, LaPO4 or La2 O3 ; Al PO3 3 or Al2 O3 , and NH4 H2 PO4 in high purity alumina crucibles in air for approximately 1 h at temperatures between 1250°C and 1500°C depending on the composition. Each batch was calcined at 500°C for 4 h, then at 900°C for 1 h, before reaching the melting temperature. The top of the crucible was covered during melting. Melts were quenched in stainless steel moulds and annealed at 600°C for 2 h. Glass compositions are divided into three groups: Series I include metaphosphate glasses (O/ P 3) with the general composition of 25 x La2 O3 ±xAl2 O3 ±75P2 O5 ; Series II glasses include glasses with general composition of 30 x La2 O3 ±xAl2 O3 ±70P2 O5 . Additional compositions were prepared to determine the glass-forming range. The compositions of selected glasses were checked by the energy dispersive analysis of X-rays (EDAX). At least ®ve spots were examined on each sample and the average composition is reported. These analyses were reproducible to 5%. 2.2. Property measurements The density (q) of the glasses was measured by Archimedes' method, using kerosene as the immersion ¯uid. Measurements were made in duplicate for each glass and the averages are reported. The refractive index (n) of glasses was measured using the Becke line method and the uncertainty in the reported values is 0:002. Glass transition temperature (Tg ) was measured by dierential thermal analysis (DTA) in platinum pans under nitrogen gas. Each sample was heated to above the anticipated Tg , then cooled at 10°C= min to 300°C to eliminate the eect of ®ctive temperature on Tg , before reheating at 10°C/min to record
Tg . These measurements are reproducible within 3°C. The average thermal expansion coecient (a) between 200°C and 500°C for each glass was determined by dilatometry with a heating rate of 5°C/ min. The estimated error in a is 3%. The chemical durability of the bulk glasses with approximate size of 1 1 1 cm3 was evaluated by weight loss measurements from glasses exposed to deionized water at 70°C for eight days. Glasses were polished to 600 grit ®nish with SiC paper, cleaned with acetone and suspended in polypropylene containers containing 100 ml of deionized water at 70°C. Measurements were conducted in duplicate for each glass and the average dissolution rate (DR), normalized to the glass surface area and the corrosion time, was calculated from the weight loss at the conclusion of the test. 2.3. Structural investigation 27
Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were collected at 94.67 MHz at a MAS frequency of 10 kHz. 1.2 ls pulses and 300 ms recycle times were used. 1 M Al NO3 3 was used as a reference for the chemical shifts. The peak maxima are reported as the peak positions. Relative site concentrations of dierent aluminum species in glasses have been taken to be the relative areas of the Gaussian curves used to deconvolute the 27 Al MAS NMR spectra. The infrared (IR) spectrum for each glass was collected between 400 and 2000 cm 1 using a Perkin±Elmer 1760X FT-IR spectrometer. Samples were prepared as pellets by pressing a mixture of glass powder and anhydrous KBr powder. The spectrometer was purged with dry nitrogen and then 20 scans were collected for each sample and the average spectrum was recorded. A pure KBr spectrum was subtracted from each glass spectrum to correct the background. 3. Results 3.1. Glass forming and properties The batch compositions of the glasses are given in Table 1. EDAX analyses of selected glasses are
Glass
Composition (mol%)
Tg °C
a 1=°C
q g=cm3
n
Log DR
75 (73.7) 75 75 (75) 75
618 631 643 685
nm 72 66 66
3.31 3.01 2.92 2.77
1.568 1.572 1.554 1.538
20 (22) 25 (27)
75 (73) 75 (73)
754 814
61 nm
2.67 2.58
1.526 1.526
nm )7.5 )7.3 No detectable wt loss )7.2 nm
0 10 (14) 15 (18.1) 20 (26.4)
70 70 70 70
(69) (66) (66.7) (64.6)
632 645 644 654
nm 65 49 46
3.42 3.12 2.95 2.79
1.604 1.576 1.552 1.530
5 (5) 0
25 (29.8) 30 (32)
70 (65.2) 70 (68)
669 754
) nm
2.65 2.52
1.518 1.512
nm Cracked )6.7 No detectable wt loss nm nm
24.375 23.75 17.5
2.5 5 15
73.125 71.25 67.5
638 640 643
72 74 50
3.27 3.25 3.01
1.576 1.574 1.562
)7 )6.9 )7.2
La2 O3
Al2 O3
P2 O5
25La75P SI-1 SI-2 SI-3
25 (26.3) 20 15 (11.5) 10
0 5 10 (13.5) 15
SI-4 25Al75P
5 (5) 0
30La70P SII-l SII-2 SII-3
30 20 15 10
SII-4 30Al70P 24.375La2.5Al 23.75La5Al 17.5La15Al a
(31) (20) (15.2) (9)
The compositions of selected glasses were analyzed by EDAX and the results are given in parentheses. The analyzed compositions and selected properties of binary La2 O3 ±P2 O5 and Al2 O3 ±P2 O5 glasses prepared as the end members of Series I and II glasses are also included for comparison.
M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
Table 1 Composition and selected properties of the LAP glassesa
213
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M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
given in parentheses. In general, some P2 O5 (< 8% relative) was lost during sample preparation. Glasses batched with 80 mol% P2 O5 had ®nal compositions similar to the metaphosphate glasses in Series I and so are not considered here. (Ultraphosphate compositions likely need to be prepared in sealed ampoules to avoid signi®cant P2 O5 volatilization [7].) The glass-forming region is given as a solid line in Fig. 1; open circles indicate glass-forming compositions and the compositions that crystallized upon quenching are represented as ®lled circles. LaPO4 and AlPO4 were present in crystallized samples of these latter La2 O3 -rich and Al2 O3 -rich compositions, respectively. Glass formation is expected to extend to the P2 O5 -corner of Fig. 1. The properties of the LAP glasses are given in Table 1. The replacement of La2 O3 and P2 O5 with Al2 O3 increases Tg and decreases a for almost all the glasses studied. The changes in Tg with Al2 O3 for Series I and II glasses are shown in Fig. 2. The increase in Tg is greater for Series I glasses than Series II glasses. The density and refractive index of the LAP glasses are given in Table 1 and shown in Fig. 3. Both density and refractive index decrease monotonically with the addition of alumina in both series. The DRs calculated from the weight loss
Volatile compositions
Fig. 2. Compositional dependence of glass transition temperature as a function of the batched Al2 O3 content for Series I and II glasses. The glass transition temperatures for the binary La2 O3 ±P2 O5 and Al2 O3 ±P2 O5 glasses (labeled `b') prepared as the end members of Series I and II glasses are included for comparison. The lines are drawn as guides for the eye.
experiments in distilled water at 70°C for eight days are given in the last column of Table 1. The DRs are in the 10 7 ±10 8 g=cm2 = min range for all the glasses studied. For SI-3 (15 mol% Al2 O3 ) and SII-3 (20 mol% Al2 O3 ) glasses, no weight loss was detectable. SII-1 glass containing 10 mol% Al2 O3 cracked during the weight loss experiment. For comparison, commercial soda±lime silicate glasses have dissolution rates in the 10 8 g=cm2 = min range in distilled water at 70°C. 3.2. Structural investigations
Fig. 1. Glass-forming region in La2 O3 ±Al2 O3 ±P2 O5 (batch composition) ternary system. Solid circles correspond to compositions that were partially or fully crystallized. The binary La2 O3 ±P2 O5 and Al2 O3 ±P2 O5 glasses (analyzed composition) prepared as the end members for Series I and II glasses are also included.
3.2.1. 27 Al MAS NMR spectra The 27 Al MAS NMR spectra for the glasses in Series I and II are given in Figs. 4(a) and (b). The spectra yield three peaks, near +33, +4, and )20 ppm. Similar peaks were observed in the 27 Al MAS NMR spectra from sodium aluminophosphate
M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
Fig. 3. Compositional dependence of density and refractive index plotted as a function of the batched Al2 O3 content for Series I and II glasses. The density and refractive index for the binary La2 O3 ±P2 O5 and Al2 O3 ±P2 O5 glasses (labeled `b') prepared as the end members of Series I and II glasses are included for comparison. Lines are drawn as guides for the eye.
(a)
215
glasses and they were assigned to Al OP4 ; Al OP5 , and Al OP6 , sites, respectively [5]. We use the same assignments for peaks observed in the LAP spectra. The positions and relative areas of the three peaks are given in Table 2. (Each peak position in the spectra from the LAP glasses is shifted 5±8 ppm up®eld from those reported for Na-aluminophosphate glasses [5], re¯ecting the greater ®eld strength of La3 on the 27 Al NMR spectra.) In Series I, the spectra are dominated by peaks representing Al(6) sites and the intensity of the peak corresponding to these sites increases with the increasing alumina content (Table 2 and Fig. 4(a)). In Series II, the spectra are dominated by peaks representing the Al(4) sites and the intensity of the peak corresponding to these sites decreases as alumina content in glass increases (Table 2, Fig. 4(b)). It has been shown that the quadrupolar eects do not cause signi®cant changes in the relative peak intensity of the 27 Al MAS NMR spectra of sodium aluminophosphate glasses [5]. Similarly, if we assume that no signal is lost due to the quadrupolar interactions, the relative 27 Al MAS NMR peak areas are proportional to the relative
(b)
Fig. 4. 27 Al MAS NMR spectra from: (a) Series I glasses [ 25 xLa2 O3 ±xAl2 O3 ±75P2 O5 , mol%]; (b) Series II glasses [ 30 xLa2 O3 ±xAl2 O3 ±70P2 O5 , mol%]. The spectra of binary glasses are included (dashed lines) for comparison.
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M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
Table 2 Chemical shifts (ppm) and relative areas (RA) of aluminum sites in LAP glasses obtained from deconvolution of spectraa Glass
Al (6)
Al (5)
Al (4)
27
Al MAS NMR
ppm
RA
ppm
RA
ppm
RA
Average Al CN
SI-1 SI-2 SI-3 SI-4 27Al73P
)20.3 )21.0 )21.3 )21.3 )21.5
0.43 0.47 0.52 0.63 0.61
4.3 2.5 3.0 3.0 2.2
0.32 0.31 0.31 0.26 0.30
33.7 32.7 32.3 32.3 32.3
0.25 0.22 0.17 0.11 0.09
5.18 5.26 5.34 5.51 5.55
SII-l SII-2 SII-3 SII-4 32Al68P
)18.1 )20.0 )20.8 )20.3 )22.0
0.21 0.24 0.27 0.32 0.42
4.8 3.5 3.5 3.0 2.0
0.31 0.33 0.35 0.36 0.36
33.7 33.2 33.2 31.1 32.0
0.48 0.43 0.38 0.32 0.22
4.72 4.81 4.89 5.00 5.25
24.375La2.5Al 23.75La5Al 17.5La15Al
)20.1 )19.3 )19.5
0.38 0.27 0.15
2.1 4.6 4.8
0.31 0.33 0.29
32.0 33.2 33.7
0.31 0.40 0.56
5.07 4.88 4.60
a
The results for two binary Al2 O3 ±P2 O5 glasses prepared as the end members of Series I and II glasses are also listed.
concentrations of the dierent Al sites and so can be used to calculate the average Al-coordination numbers (CN) given in the last column of Table 2. The average Al-CN in Series I glasses ranges from 5.2 to 5.6; for Series II, Al-CN is between 4.7 and 5.0. The eect of composition on Al-CN for both series is shown in Fig. 5. For both series, Al-CN increases with increasing Al2 O3 content. 3.2.2. IR measurements The IR spectra of glasses in Series I and II are shown in Figs. 6(a) and (b). The spectra are very similar to those reported for various metaphosphate glasses [8,9]. In general, there are ®ve major bands observed at around 1267, 1073, 912, 778, and 490 cm 1 . These bands can be assigned to the mas PO2 ; ms PO2 ; mas POP; ms POP, and dPO2 modes of PO3 n chain groups, respectively [9,10]. The intensity of strong bands observed around 1267, 1073 and 912 cm 1 for the glass containing 5 mol% Al2 O3 (Fig. 6(a)) decreases as the Al2 O3 content increases, and their frequencies shift to higher values. Similar behavior is observed for glasses in Series II (Fig. 6(b)). The overall intensity of the mas PO2 and ms PO2 bands is lower and the eect of the addition of alumina is more pronounced in the Series II glasses. The intensity of the band at 1080 cm 1 ms PO2 decreases as the
Al2 O3 content in the glass increases and it disappears totally and a new band at 1154 cm 1 emerges for the Series II glass containing 25 mol%
Fig. 5. Average Al-CN for the glasses in Series I and II determined from 27 Al MAS NMR spectra. The average Al-CN for the binary Al2 O3 ±P2 O5 glasses (labeled `b') prepared as the end members of Series I and II glasses are included for comparison. The lines are drawn as guides for the eye.
M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
(a) Fig. 6. IR spectra from: (a) Series I glasses [ 25 70P2 O5 , mol%].
217
(b) xLa2 O3 ±xAl2 O3 ±75P2 O5 , mol%]; (b) Series II glasses [ 30
Al2 O3 . This new band is assigned to the asymmetric vibrations of mas PO3 units [8±10]. 4. Discussion The addition of alumina to sodium phosphate glass strengthens the network by creating crosslinks between phosphate chains. The resulting increase in structural connectivity increases Tg , decreases thermal expansion and improves aqueous durability [5]. Similar property trends are observed for the LAP system (Table 1, Figs. 2 and 3). The decrease in thermal expansion coecient observed when Al2 O3 replaces La2 O3 is not as pronounced as the increase in Tg . Density and refractive index show similar compositional dependence as both properties decrease almost monotonically with increasing Al2 O3 content. The decreases (Fig. 3) are due to the replacement of more massive La3 ions by Al3 ions. There are no breaks in the properties as the alumina content in the glass is increased. For sodium aluminosilicate (NAS) and sodium aluminophosphate (NAP) glasses, a break in the properties is observed when Al2 O3 is added to the glass. In the NAS system, the break occurs when the [Al]/[Si] ratio in the glass exceeds 1.0, at
xLa2 O3 ±xAl2 O3 ±
which point all non-bridging oxygen (NBO) bonds (Si±O Na ) are converted into Si±O±Al(4) bonds [11]. In the NAP system, property breaks are observed when the [O]/[P] ratio exceeds 3.5, the pyrophosphate stoichiometry, and the dominant Al-CN changes from octahedral to tetrahedral [5]. No such breaks in properties (or Al-CN) are observed for the LAP glasses investigated in this study primarily because aluminophosphate glasses with such high O/P ratios could not be made with La2 O3 . The intensities of the IR bands assigned to the PO2 units decrease with increasing concentrations of Al2 O3 in Series I (Fig. 6(a)); however, there are no signi®cant structural changes to the metaphosphate network and there is no evidence for the presence of signi®cant concentrations of terminal groups (mas PO3 at 1154 cm 1 ). These observations are consistent with the analyzed compositions which yield O/P ratios of 3:0. The glasses in Series II, on the other hand, have O/P ratios of 3.3 and so possess structures with signi®cant concentrations of terminal PO3 groups as indicated by the presence of the IR band at 1154 cm 1 (Fig. 6(b)). The observed increase in the frequencies of the asymmetric and symmetric stretching PO2 bands and the decrease in their intensities with increasing
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M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219
Al2 O3 content, for both Series I and II, may be attributed to greater electronegativity of Al3 (1.5) compared to that of La3 (1.1) [12]. The greater electronegativity makes the P±O bond more ionic by increasing the charge separation between phosphorus and oxygen ions which decreases the intensity of the vibration. Peak intensity is inversely proportional to frequency because the electronegativity, which aects the vibrational force constant, and the size of the cation, which aects the P±O bond angle, have opposite eects on the PO2 frequency [13]. According to the vibrational model of Rouse et al., the ms PO2 shifts to higher frequency while mas PO2 shifts to lower frequencies when larger cations are replaced by smaller cations [14]. However, in the IR spectra shown in Fig. 6, both ms PO2 and mas PO2 bands shift to higher frequencies, indicating that the metal± oxygen force constant has the dominant eect on the structure of these glasses. The structural roles of La3 and Al3 in these glasses will be dierent. This can be better understood by comparing the structures of binary aluminum and lanthanum phosphate crystals and glasses. Crystalline La PO3 3 has a structure based on in®nite chains of corner-sharing PO4 tetrahedra and edge-sharing LaO8 dodecahedra [15]. In contrast, the Al3 ions in crystalline Al PO3 3 do not share common oxygens; every Al(6) is isolated by P±O±Al(6) bonds to the in®nite metaphosphate chains [16]. Neutron and X-ray diraction studies of lanthanum metaphosphate glass show that the La±O-CN is 7 [17]. This exceeds the number of terminal oxygens (6) that are available to coordinate each La3 cation, leading to the clustering of the LaOn polyhedra. In the structures of binary aluminophosphate glasses, on the other hand, tetrahedral Al replaces octahedral Al with increasing Al2 O3 content in order to avoid the formation of Al±O±Al bonds [18]. A recent multidimensional NMR study of aluminum containing phosphate glass-ceramics con®rms this `aluminum avoidance' behavior [19]. The increase in the average Al-CN as Al2 O3 replaces La2 O3 in both Series I and II (Fig. 5) can be explained by considering the respective coordination environments of Al3 and La3 in a phosphate glass. If we assume that the La-CN is
7, as reported for La PO3 3 glass [17], then the La3 ions must use most of the NBOs available in the glass, leaving fewer NBOs to coordinate the Al3 ions; hence, Al-CN is low. As Al2 O3 replaces La2 O3 , greater numbers of NBOs (per trivalent cation) are made available and the average Al-CN increases. The average Al-CN is lower for Series II glasses than Series I glasses (Fig. 5) because the former series has a fewer NBOs per trivalent cation and the lower Al-CN is necessary to avoid Al±O±Al linkages.
5. Summary Glass-forming characteristics, properties and structure of glasses in the La2 O3 ±Al2 O3 ±P2 O5 system have been investigated. Tg increases and thermal expansion coecient, refractive index, and density all decrease as the alumina content increases. IR spectroscopy reveals that the glass networks are dominated by bridging P-tetrahedra that constitute the metaphosphate chains, with some chain-terminating sites in glasses with O/P ratios > 3:0. 27 Al MAS NMR reveals that Al3 ions are incorporated into the phosphate networks as 4-, 5-, and 6-coordinated sites. The average AlCN depends on the Al/La and O/P ratios and changes with composition to avoid the formation of Al±O±Al linkages.
Acknowledgements Acknowledgement is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research. The authors thank Clarissa Vierrether, for providing the EDAX data, and Holly Bentley, for assistance in the preparation and characterization of the LAP glasses. References [1] J.H. Campbell, T.I. Suratwala, J. Non-Cryst. Solids 263&264 (2000) 318. [2] Y.B. Peng, D.E. Day, Glass Technol. 32 (1991) 166.
M. Karabulut et al. / Journal of Non-Crystalline Solids 283 (2001) 211±219 [3] R.K. Brow, L. Kovacic, R.E. Loehman, Ceram. Trans. 70 (1996) 177. [4] D.E. Day, Z. Wu, C.S. Ray, P. Hrma, J. Non-Cryst. Solids 241 (1998) 1. [5] R.K. Brow, R.J. Kirkpatrick, G.L. Turner, J. Am. Ceram. Soc. 76 (1993) 919. [6] A. Kishioka, M. Hayashi, M. Kinoshita, Bull. Chem. Soc. Jpn. 49 (1976) 3032. [7] H.D. Park, E.R. Kreidler, J. Am. Ceram. Soc. 67 (1984) 23. [8] G.J. Exarhos, P.J. Miller, W.M. Risen Jr., J. Chem. Phys. 60 (1974) 4145. [9] J.J. Hudgens, S.W. Martin, J. Am. Ceram. Soc. 76 (1993) 1691. [10] S. Bruni, F. Cariati, D. Narducci, Vib. Spectrosc. 7 (1994) 169. [11] W. Tasker, D.R. Uhlmann, P.I.K. Onorato, M.N. Alexander, C.W. Struck, J. Phys. 46 (1985) C8-273.
219
[12] L. Pauling, in: The Nature of the Chemical Bond, 3rd Ed., Cornell University, Ithaca, NY, 1960. [13] B.N. Nelson, G.J. Exarhos, J. Chem. Phys. 71 (1979) 2739. [14] G.B. Rouse Jr., P.J. Miller, W.M. Risen Jr., J. Non-Cryst. Solids 28 (1978) 193. [15] J. Matuszewski, J. Kropiwnicka, T. Znamierowska, J. NonCryst. Solids 290 (1988) 285. [16] H. van der Meer, Acta Crystallogr. B 32 (1976) 2423. [17] U. Hoppe, R. Kranold, D. Stachel, A. Barz, A.C. Hannon, J. Non-Cryst. Solids 232&234 (1998) 44. [18] R.K. Brow, C.A. Click, T.M. Alam, J. Non-Cryst. Solids 274 (2000) 9. [19] P. Hartmann, J. Vogel, U. Friedrich, C. Jager, J. NonCryst. Solids 263&264 (2000) 94.