LATTICE-DYNAMICS OF ZEOLITIC SILICA POLYMORPHS

Rigid ion and shell model calculations of the infrared and Raman active modes of α-quartz, aluminum-free sodalite, and dealuminated faujasite are presented. Results of the rigid ion calculations show poor agreement with experimental data but the agreement significantly improved when using shell model calculations. A group theoretical analysis of the spectra enables an analysis in terms of dominantly stretching, bending, or torsional modes. A rather good agreement of the frequencies in the bending and torsional mode regions is found. One concludes from comparing with the stretching modes that the short range potential for the Si-O stretch frequency is too weak. The improvement of the shell model versus the rigid ion model results can be ascribed to screening of the long range electrostatic interactions by polarization of the O2- anions. Calculated frequency splittings between transversal and longitudinal optical modes indicate that the use of partial ionic charges, in combination with an improved short range potential, might result in a better simulation of the infrared and Raman spectra. © 1990 American Chemical Society.