Space group symmetry and Al—O—P bond angles in AlPO4-5

Abstract

Static lattice simulations are presented on the microporous aluminophosphate AlPO₄-5. Minimisation methods together with lattice dynamical calculations are able to identify stable minima for the structure. We show, in agreement with the recent work of Henson et al., that reduction of the symmetry of the structure from P6cc to P6 leads to the relaxation of Al—O—P angles from the linear structure reported in earlier crystallographic structures. We find that the orthorhombic space group Pcc2 suggested recently by Mora et al. has only a very slightly higher energy than that calculated for the P6 structure. Differences in bond lengths and angles between the two structures are correspondingly small. We find that the inclusion of a representation of the polarisability of the oxygen in the potential model is essential in removing the linear Al—O—P angles in the simulated structures.