MODELING OF STRUCTURE AND VIBRATIONAL-SPECTRA OF ALPO4-5 AND ITS SILICA ANALOG SSZ-24

In this study, the structural and vibrational properties of the AlPO4 Structure AlPO4-5 and the silica structure SSZ-24 are compared. Lattice energy calculations are done using existing potential parameter sets suitable for silicas and AlPO4's. For the computation of vibrational spectra of silica systems, force constants derived by Etchepare et al. are used. For AlPO4 spectra simulations, a new force field is presented that is based on a fit on vibrational frequencies of alpha-berlinite, the AlPO4 analog of alpha-quartz. Lattice energy calculations result in a symmetry of AlPO4-5 and SSZ-24 that is lower than derived experimentally. A shift of layers is observed for both structures when a potential with partial charges is used. This shift is not found for a formally charged shell model calculation of SSZ-24. These results are indicative for an underestimation of the charges used in the partial charge model. The influence of structure on spectra is shown to be rather weak. The main differences between the spectra of AlPO4-5 and SSZ-24 are due to the interatomic force constants.