CONSTRAINTS ON THE STRUCTURE AND DYNAMICS OF THE BETA-CRISTOBALITE POLYMORPHS OF SIO2 AND ALPO4 FROM P-31, AL-27 AND SI-29 NMR-SPECTROSCOPY TO 770 K

Nuclear magnetic resonance spectroscopic data are presented for the cristobalite polymorphs of AlPO4 and SiO2 from RT to 770 K, through their respective alpha-beta transitions. The nuclear magnetic resonance (NMR) data include chemical shifts for P-31, Al-27, and Si-29, Al-27 quadrupole coupling parameters, and P-31 and Al-27 spin-lattice relaxation rates. Also presented are electron diffraction patterns of beta-cristobalite AlPO4 that show diffuse scattering similar to that reported previously for SiO2. For the alpha-phases of both AlPO4 and SiO2, the chemical shifts decrease approximately linearly with increasing temperature from RT to T-c and discontinuously by -2 to -3 ppm from alpha to beta. This result is consistent with a small, continuous increase in the mean T-O-T angle (<theta>) of the alpha-phases with increasing T and an increase of <theta> by about 4 degrees across the alpha-beta transition for both cristobalite and its AlPO4 analogue. Based on the Si-29 chemical shifts, the mean Si-O-Si angle for beta-cristobalite is 152.7+/-1 degrees near T-c. For AlPO4-cristobalite, the Al-27 nuclear quadrupole coupling constant (C-Q) decreases approximately linearly from 1.2 MHz at RT to 0.94 MHz near T-o (493 +/- 10 K). At the alpha-beta transition the Al-27 C-Q approaches zero, in agreement with the cubic average structure observed by diffraction. The satellite transitions retain a small frequency distribution above the alpha-beta transition from electric field gradients attributed to defects. The short-range cubic symmetry of the Al-site and non-linear Al-O-P angle support a dynamically disordered model of the beta-cristobalite structure. Complete averaging of the Al-27 quadrupole coupling in the beta-phase indicates that the lifetime of any short-range ordered domains must be shorter than about 1 mu s.