Constraints on the structure and dynamics of the β-cristobalite polymorphs of SiO2 and AlPO4 from 31P, 27Al and 29Si NMR spectroscopy to 770 K

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