SPACE GROUP SYMMETRY AND AL-O-P BOND ANGLES IN ALPO4-5

Static lattice simulations are presented on the microporous aluminopho sphate AlPO4-5. Minimisation methods together with lattice dynamical c alculations are able to identify stable minima for the structure. We s how, in agreement with the recent work of Henson et al., that reductio n of the symmetry of the structure from P6cc to P6 leads to the relaxa tion of Al-O-P angles from the linear structure reported in earlier cr ystallographic structures. We find that the orthorhombic space group P cc2 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 smal l. We find that the inclusion of a representation of the polarisabilit y of the oxygen in the potential model is essential in removing the li near Al-O-P angles in the simulated structures.