16455
J. Phys. Chem. 1995,99, 16455-16459
Structure and Dynamics of CaA1204 from Liquid to Glass: A High-Temperature 27Al NMR Time-Resolved Study Dominique Massiot,* Dominique Trumeau, Bruno TOUZO, Ian Farnan, Jean-Claude RiMet, AndrC Douy, and Jean-Pierre Coutures CRPHT-CNRS, 45 071 OrlCans Cedex 2, France Received: May 23, 1995; In Final Form: August 16, 1995@
The free cooling of an aerodynamically levitated liquid CaA1204 droplet from 2400 K to supercooled liquid and glass in a few seconds has been monitored by time-resolved 27AlNMR. The containerless setup avoids heterogeneous nucleation and allows CaA1204 liquid to vitrify with an average cooling rate of 200 K s-I. In all the observed temperature range, the 27Alspectra of the liquid phase have Lorentzian line shapes with line widths of a few hundred hertz which have been verified as being due to the aluminum relaxation TI time in the extreme narrowing regime. The 27Alchemical shift of the liquid sample increases linearly with decreasing temperature between 2400 and 1700 K (ddldT = -6.0 p p d 1 0 0 0 K negative slope). The observed isotropic chemical shift position of the glass measured by MAS NMR at room temperature but plotted at Tg (measured by differential scanning calorimetry) falls on this straight line. This continuous evolution is attributed to the progressive dissociation of the A104 tetrahedral network of the glass to form A105 and A106 in the liquid with an increase of the mean coordination number of 0.2 per 1000 K, in agreement with previous ion dynamic simulations. Assuming a quadrupolar relaxation mechanism for 27Al, the correlation time can be described as a function of temperature. It closely matches the correlation time derived from macroscopic shear viscosity measurements with the same temperature dependence (in this range 1700-2400 K). They are both related to the same microscopic fluctuations.
Introduction
Experimental Section
The temary system Si02-Al203-CaO is of particular importance for material sciences and glass, ceramics, or cement industries which often have to deal with vitrification and crystallization processes from the high-temperature liquid. At high temperature, nuclear magnetic resonance remains sensitive to the structure and the dynamics of liquid oxides.' With a containerless system where the sample is laser heated and levitates on a gas jet, placed in an NMR probe it is possible to observe liquids at very high temperature (T > 2000 K).2,3 The relaxation times, obtained from the line width, assuming a fully averaged liquid state, give microscopic hints to the dynamics of the high-temperature liquid that have been proposed to be related to macroscopic properties like shear viscosity in the light of ion dynamic simulation^.^-^ Recent improvements in the NMR levitation setup made it possible to follow in real time the cooling process of liquid alumina from 2800 K down to crystallization by time-resolved 27AlNMR.7 Cd1204 is one of the crystalline phases of the binary A1203-Ca0 join, with a congruent melting point at 1878 K. It has been previously studied by 27AlNMR in crystalline powders,*-I0 glasses, and even liquids at high t e m p e r a t ~ r e . ~ ~ ~The - " ~Cd1204 '~ liquid usually crystallizes but can vitrify when cooled in containerless conditions, which is the case for the aerodynamic levitation device that is used in our high-temperature NMR setup. With the latest improvements to the hardware we can accurately stabilize the temperature of the liquid sample, carry out relaxation time measurements, and acquire time-resolved 27Al spectra with known temperature, thus getting structural and dynamic insights to the cooling process from the stable liquid to the supercooled system and the glass.
The CaA1204 sample was prepared from molar solutions of A1(N03)3 and Ca(N03)~. A stoichiometric mixture is spraydried at 210 "C, giving a homogeneous and thin nitrate powder which is first calcined at 700 "C to decompose the nitrates and then heated to 1500 "C giving a crystalline powder of CaA1204. The crystallinity is checked by X-ray diffraction. The glassy spherical sample (2-3 mm, -50 mg) was prepared by aerodynamic levitation from pressed powder in a separate levitation device. Figure 1 presents the NMR experimental setup. The sample is placed in a convergent-divergent boron nitride nozzle in which it levitates on an adjustable air jet. The heating power is now supplied by two computer controlled CW COZ lasers (15-120 W) from the top and the bottom of the cryomagnet, instead of a single focused laser from the top in previous settings7 This ensures a better thermal homogeneity of the levitating sample. With the available laser power a CaA1204 sample can be heated to liquid phase up to 2400 K. The NMR measurements were acquired with a Bruker MSL-300 spectrometer using a vertical probe tuned for 27Al (78.2 MHz at 7.04 T) fitted to take the levitation device within its Helmholtz radio-frequency coil (20 mm diameter). With this setup, the 27AlNMR signal of a liquid CaA1204 droplet (mp = 1878 K) is a narrow Lorentzian line with a typical line width of 100150 Hz. It is well-resolved ( S I N 10) with a single scan acquisition. All the spectra are referenced to the resonance of 27A1in a 1 M AI(N03)3 solution measured at room temperature in a glass spherical bubble placed at the position of the levitating sample. The temperature is measured with a commercial pyrometer working at A = 0.85 pm equipped with an optical fiber. The end of the fiber is positioned at 75 mm above the sample inside the magnet. With this setting the solid angle sampled by the
* Corresponding author. E-mail
[email protected]. @Abstractpublished in Aduunce ACS Abstracts, October 1, 1995. 0022-365419512099-16455$09.00/0
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0 1995 American Chemical Society
16456 J. Phys. Chem., Vol. 99, No. 44, 1995
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Massiot et al.
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RF coil BN levitator laser beam
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ZnSewindow
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lasers Figure 1. High-temperature NMR experimental setup. The sample is levitated on a gas jet and heated by two CW COz lasers inside the NMR probe. 100%
laser power 2400 2200 2000
1800
1600
T
1
0 80
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~
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Figure 2. Schematic presentation of the time-resolved experiment. Squares report 27Al chemical shifts (6) obtained by simulation of the
successively acquired spectra. Diamonds report the calibrated pyrometric temperature ( r ) of the sample. The laser power is shut down 1 s after the start of acquisition. pyrometer is greater than the sample itself. The temperature calibration has been done outside the magnet on a setup having the same geometry and equipped with calibrated pyrometers at different ~ave1engths.l~ The detected temperature range is from 2400 to 1700 K. The relative variations of temperatures are well resolved while the absolute temperature measurements are reliable within &50 K. With the improved stability and homogeneity of the heating power, we could carry out some spin-lattice ( T I )and spinspin (T2)relaxation time measurements at constant temperature by usual inversion recovery and Hahn echo (n12-z-n) sequences, respectively. As the NMR signal of CaAl204 can be recorded in one scan and as its relaxation time is short, the measurements can be repeated rapidly (50 ms) as previously described for liquid A1203.7 It is thus possible to trace the cooling of the sample from 2400 K in a few seconds when the laser power is shut down. Figure 2 gives a schematic representation for the timing of the experiment together with the pyrometric temperature and the NMR line position (shift) as function of time. To record the experimental data, the computer used for controlling the laser power is also used to trigger NMR measurement (external trigger on the spectrometer) and acquire the pyrometer signal. The laser power is shut down after a controlled delay (1 s) to
300
100
200
0 @Pm)
-100
-200
-300
A
-400
Figure 3. Central part of the 27AlMAS (14 kHz) NMR spectrum of the CaA1204 glass (A) the experimental spectrum and (B) the modeled spinning sidebands of the (3/2) satellite transitions. The 6R ; and are measured at 85.3 and 39.8 ppm, respectively.
check the starting steady state regime. The typical acquisition conditions are n / 2 (37 ps) flip angle, 50 ms recycle time, and 128 individual spectra, for a total experiment duration of 6.4 s. The signal is constant before the laser shutdown and evolves during the cooling of the sample. It is important to note that the signal of the solid is never detected with these experimental conditions. All the experiments were carried out on the same sample (48 mg, 3.1 mm diameter). A small weight loss of 2% was observed during the full experimental session. The experimental measurements are highly reproducible in terms of pyrometric temperature (starting and cooling) as well as collected NMR data. As the acquisition of one spectrum takes less than 26 ms, this corresponds to a maximum temperature evolution of 15 K for the highest cooling rate (-600 Ws at 2400 K) at the beginning of the experiment. All the spectra are well fitted with one Lorentzian line giving the line position (isotropic shift or diso), the full width at half-maximum (fwhm), and the integrated magnetization as functions of time and temperature. At the lowest temperatures the results obtained from fitting scatter as the line becomes much wider, and the signal-to-noise ratio decreases. A complementary differential scanning calorimetry (DSC) analysis has been carried out to measure the glass transition (T,) on glassy droplets (Setaram DSC-1600)between room temperature and 1375 K at 10 and 3 K min-' rising slope. The glass sample coming from the levitation experiment has been characterized by 27Alhigh-speed MAS (14 kHz) NMR. The central part of the MAS spectrum (Figure 3) shows the typical asymmetric broad line with trailing right edge corresponding to the central ( - I l l l/2) transition and the spinning sidebands arising from the outer (f3I2 f l 1 2 ) transitions while f3I2) transitions are too wide to be detected. the (&5/2 According to previously discussed protocol^,^^^'^^'^ the isotropic chemical shift is estimated as
- -
-
-
-
where d;f2 and dyf2 represent the center of gravity of the (f3/2 f l / 2 ) and (-'I2 ' 1 2 ) transitions, respectively. The d\h value is measured from the position of the hidden N = 0 spinning sideband modeled from the observed outer spinning sidebands and the spinning rate. The dff2value is estimated by integration from the apparent center of gravity of the central line. The estimated quadrupolar product CQ, can thus be given asI5
J. Phys. Chem., Vol. 99, No. 44, 1995 16457
Structure and Dynamics of CaA1204
supercooled liquid
8 @pm)
where I is the nuclear spin number, YO is the Larmor frequency, and V Q is the asymmetry of the quadrupolar interaction tensor. In these computations several assumptions or approximations are made and have to be discussed further. If d;f2 is precisely measured with the sharp spinning sidebands (better than f 0 . 5 ppm), the dff2is only an estimate. As the trailing asymmetric line shape of the observed line can be modeled as a Gaussian distribution of the quadrupolar product CQ, around a mean value, the signal/noise of the high coupling part of this distribution is much lower than that of the low coupling part, due to the secondorder spreading which is proportional to the squared quadrupolar product, and part of it can vanish in the base line with overlapping spinning sidebands. Consequently, the measured apparent center of gravity is only a measurement by excess of the true value. The errors made on the determination dif2 have rather small influence on the determination of the diso value (one-ninth of the error made, according to eq 1) while it has direct influence on the estimated value of C Q ~the , quadrupolar product (eq 2) which is thus known to be a minimum.
Results and Discussion Temperature and "Al Chemical Shift. After the laser shutdown the integrated magnetization from the liquid sample is slightly increasing with decreasing temperature according to a Curie law. This temperature dependence is too small to be further used for temperature measurement as previously proposed for liquid alumina? The observed scattering of the measured magnetization at the end of the acquisition (T < 1800 K) is due to the progressive loss of resolution (signdnoise) with the enlargement of the NMR line (observed for a single scan) that disappears in the noise. All the cooling experiments start from the high-temperature levitated liquid at 2400 K heated with the full laser power. Under those conditions the sample cools at an average rate of 200 K s-l. For this CaA1204 composition we always observe the vitrification of the sample thanks to the absence of heterogeneous nucleation in contactless conditions. During the cooling the observed chemical shift is continuously increasing from 74 to -79 ppm (when the signal vanishes) with decreasing temperature (negative dd/dT slope). Figure 4 reports this behavior and the progressive enlargement of the line. These chemical shift values are in agreement with the value of 77 ppm previously reported for the same composition.6,'6 The chemical shift evolution is going slowly to less shielded state and toward the 27Alisotropic chemical shifts observed for the tetrahedrally coordinated sites of crystalline CaA1204 (six different tetrahedral sites from 81 to 86 p ~ m ) . ~ , ~ The 27AlMAS NMR experiment carried out on the final glass droplet evidences a single AlIv contribution with the usual asymmetric shape for the central transition and a single set of outer spinning sidebands. For neighboring compositions previous studiesI7 clearly detected 1% A106 contributions, we can thus assume to have more than 99% A104 in our sample. We measure for this single contribution at 85.3 ppm and d;f2 at 39.8 ppm. This gives, according to eqs 1 and 2, an isotropic position disoat 80.2 ppm and a quadrupolar product CQ,at least equal to 6.4 MHz. By DSC analysis carried out on similar samples (droplets cooled from levitated liquids), we measured a glass transition temperature Tgof 1180 & 4 K. This value is
80
liquid
I'
70
mp 60 I100
1300
1500
1700
1900
, 2100
2300
T(K)
2500
Figure 4. Evolution of the 27Al chemical shift (6) versus the temperature (r). Open squares represent the data obtained during the cooling, and solid square represents the 6,,, value derived from the MAS spectrum of the glass. The straight dotted line has a slope of d6/dT = -(6.0 & 0.3) ppm/lOOO K. The bars give the line width of the liquid line.
in good agreement with a previous measurement of 1178 KI8 and falls in the extrapolation of the data reported on the CaOrich side of the CaO-Al203 binary system.I7 We can plot in a single diagram the "Al shifts obtained for the high-temperature liquids and from the glass taken to represent the liquid frozen at Tg.I9 These data exhibit a linear correlation through the whole range of temperature (1180-2400 K), as shown in Figure 4 with a negative slope of dd/dT = -(6.0 & 0.3) ppm/lOOO K. This negative slope is opposite to the positive slope of 2.5 ppm/lOOO K previously observed during the cooling of liquid alumina7 that was interpreted by analogy with the isostructural temperature evolution of 25Mgin crystalline Mg020 as a pure thermal expansion effect evidencing no significative change in the structure with the temperature for the "fragile" ionic alumina liquid.21%22 Negative slope thermal dependence of the chemical shift was also reported for 27Alin liquids (two or three points measured at Tg and in the related liquids between 1200 and 1350 "C) for some sodium and lithium aluminosilicates characterized by a more polymerized netw0rk.2~ Previous studies carried out by 27Al N M R and Raman spectroscopy on glasses and liquids of the CaO-Al203 system, compared to ion dynamics computations, have shown that those liquids consisted of Alw, Alv, and A l v ~polyhedra even at vibrational time scales.6 The CaA1204liquid is located on the compensation line ( A K a = 2) and is in fact expected to be the most polymerized system of the CaO-A1203 binary system with all the aluminum possibly placed in tetrahedral copolymerized Q4 type units (like in the crystalline CaAl2O4) structure. Going to the CaO-rich side the network is depolymerized by addition of alkaline earth (like in Ca3A1206 or Ca12Al14033),and going to the Al-rich side the aluminum tetrahedral network is no longer compensated and A1 has to go in 5- or 6-fold coordinated positions (like in CaAI12019). In agreement with previous experiments,6the 27Al MAS N M R spectrum observed for the glass only evidences tetrahedrally coordinated aluminum showing that the glass at freezing temperature Tgand with no heterogeneous nucleation is fully polymerized. The observed differential slope of -6.0 - 2.5 = -8.5 ppm/lOOO K (with -2.5 for isostructural evolution) is thus interpreted as tracing the progressive dissociation of the network from A l ~ v(at Tg)to progressively form Alv and AlvI with an increasing average coordination number of 0.2 per 1000 K. This is consistent with the previously published6 ion dynamics simulations on this same composition, giving a AlIv/Alv ratio of -3/2 at 3000 K and reporting an increasing coordination number with increasing temperature between 2700 and 3600 K.
Massiot et al.
16458 J. Phys. Chem., Vol. 99, No. 44, 1995 5
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Figure 5. Evolution with temperature of the correlation times derived from 27AlNMR relaxation time measurement tc(empty squares) and from shear viscosity ts(full circles). The line represents a modeling of t~ according to ref 27.
Relaxation Rates. The TI and T2 relaxation times of 27Al in the liquid state have been measured at constant temperature. This takes approximately 10 min, and the experimental conditions (probe tuning, levitation regime, laser power) appeared to be “reasonably” stable throughout the experiment. For both TI and T2 we observed an exponential evolution. At 2400 K we measured TI and T2 of a few milliseconds, in agreement with the observed line width at half-maximum of -120 Hz (much greater than the field inhomogeneity) corresponding to the liquid averaged regime TI TZ FZ T2* = 2.6 ms. This was an experimental proof required to validate previous assumptions made to interpret high-temperature 27AlNMR results?-6 Given these short relaxation times the 50 ms recycle time is much greater than ~ T Iand , the obtained results are truly independent measurements of the relaxed spin system. The further reported relaxation times are deduced from line width measurements. The quadrupolar interaction being the main perturbation to the Zeeman order for 27Al,we assume a relaxation mechanism by fluctuation of the quadrupolar interaction, with an exponential correlation function,24 the relaxation time TI is linked to the dynamic quadrupolar product CQ, = C Q G and the correlation time zc of this fluctuation following eq 3:
-1_ -- 3 2 TI
21+3 2z 10 Z2(2Z- 1) Qa c
(3)
The dynamic quadrupolar product CQ, has to be known25to derive z,. Only the static quadrupolar product can be measured in related solid state compounds where it ranges from -3 MHz in crystalline CaA12049 to -10 MHz for the highest static quadrupolar coupling constant observed in crystalline phases of the Ca0-&03 join (Ca3A12069) or even 15 MHz for the highest coupling constant observed in inorganic nonprotonated crystalline phase.26 This implies a possible range of more than 1 order of magnitude for the squared quadrupolar coupling product and thus the derived correlation times. For the glass sample (obtained from the levitation device) we measured a CQ, value of at least 6.4 MHz by 27Al high-speed MAS experiment (see discussion above) that we will use to further compute zc. The macroscopic shear viscosity vs has been previously experimentally measured on CaAl2O4 liquid at high temperature by U r b a i ~ *This ~ shear viscosity is directly related to its characteristic time z, according to the Maxwell law zs= vS/G, where G, is the shear modulus at infinite frequency (log(G,) = 10.0 f 0.5).’9,21,22 Figure 5 reports jointly the characteristic times z, (taking log(G,) = 10) the measured shear viscosity (full circles) with
their modeling (curve) according to Urbain27 and the 27Al relaxation correlation times tc computed directly from the observed line width and the average quadrupolar product CQ, = 6.4 MHz measured on the glass. The two sets of data closely match the same orders of magnitude and temperature dependence which clearly evidences once more that the fluctuation mechanism inducing NMR quadrupolar relaxation of 27Aland the viscosity thus have to be related to the same microscopic fluctuation in the liquid and the supercooled liquid. When reporting the same data in a ln(z) versus 1000/T (K) diagram (inset in Figure 5) both sets of data plot as straight lines with slightly different slopes. The computed Arrhenius activation energies are 189 kJ mol-’ for the viscosity2’ and 142 kJ mol-’ for the 27Al relaxation. These straight lines do extrapolate far below Tg (1 180 f 4 K) for a correlation time of lo2 s; this is characteristic of a fragile liquid. It should be noted that the exact G, value and its thermal dependence are not precisely known,19~21322 and a slight increase of G, or an increase of the dynamic quadrupolar product CQ, would lead to a nearly perfect match of the two sets of data. An increased quadrupolar product (8.5 MHz instead of 6.4 MHz) would in fact very well agree with the observed somewhat higher quadrupolar coupling in hyperquenched glasses of neighboring compositions.
Conclusion Time-resolved 27AlNMR data have been acquired during the cooling of a liquid droplet of CaA1204composition from 2400 to 1700 K (in -5 s) before vitrification in contactless conditions. The temperature dependence of the position and the line width of the observed single Lorentzian-shaped line is interpreted as following the thermal evolution of the local structure and the dynamics of aluminum in this very high-temperature liquid. The chemical shift exhibits a negative temperature dependence (dd/ dT = -(6.0 f 0.3) ppd1000 K) consistent with the isotropic chemical shift measured on the glass and reported at T, measured on the same samples. This thermal dependence is interpreted as characteristic of a progressive increase of the averaged aluminum coordination from -100% A l ~ vat T, with a rate of 0.2 units per 1000 K. The line width of the 27Alsignal has been checked to represent the spin-lattice relaxation time of the 27Al spin system. The relaxation correlation time zc, computed assuming a relaxation mechanism by fluctuation of the quadrupolar interaction, is in good agreement with the correlation time of viscous flow z, and shows a similar temperature dependence. Despite the small mismatch between their activation energy, these two correlation times, derived from microscopic (zc) and macroscopic (z,) measurements, can be understood as linked to the same fluctuation involving the aluminum coordination changes as in more viscous silicate systems. Further studies are currently in progress to extend the compositional range to the CaO-Al203 binary system and to study the influence of cooling rates or redox conditions. This should lead to a better understanding of the high-temperature liquid properties, vitrification, and liquidsolid transition in these aluminate systems.
Acknowledgment. We thank Christian Brkvard (Bruker, France) and Detlef Muller (Bruker Karlsruhe, Germany) for supporting the development of high-temperature laser-heated probe heads. We also thank Y. Auger and H. Chaudret for technical support in modifying the probe head and the laser power control. We acknowledge financial support from CNRS ( U p 4212) and RCgion Centre.
Structure and Dynamics of CaA1204
References and Notes (1) Stebbins, J. F. Chem. Rev. 1991, 91, 1353. (2) Taulelle, F.; Coutures, J. P.; Massiot, D.; Rifflet, J. C. Bull. Magn. Reson. 1989, 11 (3-4), 318. (3) Coutures, J. P.; Massiot, D.; Bessada, C.; Echegut, P.; Rifflet, J. C.; Taulelle, F. C. R. Acad. Sci. 1990, 310, 1041. (4) Poe, B. T.; McMillan, P. F.; Cote, B.; Massiot, D.; Coutures, J. P. J. Phys. Chem. 1992, 96, 8220. (5) Poe, B. T.; McMillan, P. F.; Cote, B.; Massiot, D.; Coutures, J. P. Science 1993, 259, 786. (6) Poe, B. T.; McMillan, P. F.; Cote, B.; Massiot, D.; Coutures, J. P. J. Am. Ceram. SOC. 1994, 77, 1832. (7) Florian, P.; Massiot, D.; Poe, B. T.; Faman, I.; Coutures, J. P. Solid State NMR, in press. (8) Muller, D.; Gessner, W.; Samoson, A.; Lippmaa, E.; Scheler, G. Polyedron 1986, 5, 775. (9) Skibsted, J.; Henderson, E.; Jakobsen, H. J. Inorg. Chem. 1993, 32, 1013. (10) Massiot, D.; Muller, D.; Florian, P.; Coutures, J. P. 12th European Experimental NMR Conference, Oulu, Finland, June 5- 10, 1994. (11) Cot6, B.; Massiot, D.; Poe, B. T.; McMillan, P. F.; Taulelle, F.; Coutures, J. P. J. Phys. (Paris) 1992, 2, C2-223. (12) Massiot, D.; CotC, B.; Taulelle, F.; Coutures, J. P. Application of NMR to Cement Science; Colombet, P., Grimmer, A. R., Eds.; Gordon and Breach Science Pulbishers: 1994; pp 153-169. (13) Coutures, J. P.; Rifflet, J. C.; Florian, P.; Massiot, D. Rev. Int. Hautes Temp. Refract. 1994, 29, 123.
J. Phys. Chem., Vol. 99,No. 44, 1995 16459 (14) Jager, C.; Kunath, G.; Losso, P.; Scheler, G. Solid State NMR 1993, 2, 73. ( 1 5 ) Massiot, D.; Muller, D.; Hubert, Th.; Schneider, M.; Kentgens, A. P. M.; Cote, B.; Coutures, J. P.; Gessner, W. Solid State NMR, in press. (16) Cot6, B.; Massiot, D.; Taulelle, F.; Coutures, J. P. Chem. Geol. 1992, 96, 367. (17) Shelby, J. E.; Shaw, C. M.; Spess, M. S . J. Appl. Phys. 1989, 66, 1149. (18) Badets, M. C. Thesis at University of Orl6ans France, July 1, 1991. (19) Dingwell, D. Eur. J. Miner. 1990, 2, 427. (20) Fiske, P. S.; Stebbins, J. F.; Faman, I. Phys. Chem. Miner. 1994, 20, 587. (21) Angell, C. A. J. Non-Cryst. Solids 1988, 102, 205. (22) Angell, C. A. J. Non-Cryst. Solids 1991, 131, 13. (23) Stebbins, J. F.; Faman, I. Science 1992, 255, 586. (24) Abragam, A. Principles of Nuclear Magnetism; Clarendon Press: Oxford, 1961. (25) Petit, D.; Korb, J. P. Phys. Rev. B 1988, 37, 5761. (26) Alemany, L. B.; Massiot, D.; Sherriff, B. L.; Smith, M. E.; Taulelle, F. Chem. Phys. Lett. 1991, 177, 301. (27) Urbain, G. Rev. Int. Hautes. Temp. Refract. 1983, 20, 135. (28) McMillan, P. F.; Petuskey, W. T.; Cote, B.; Massiot, D.; Landron, C.; Coutures, J. P. Submitted to J. Non-Cryst. Solids. JP951422B
