A coupled electron diffraction and rigid unit mode (RUM) study of the crystal chemistry of some zeotypic AlPO 4 compounds

Abstract

Electron diffraction and lattice dynamical calculations are used to investigate the unit cells, space group symmetries and inherent displacive flexibility of the room-temperature average structures of AlPO 4-8, AlPO 4-16 and AlPO 4-tridymite. The zero-frequency rigid unit modes (RUMs) of the idealized high-symmetry polymorphs thereof are also investigated along with their relationship to the lower-temperature polymorphism of these zeotypic aluminophosphates. The clear presence of G ± 1 2 c * satellite reflections in addition to the Bragg reflections ( G) of the underlying Cmc2 1 parent structure in the case of AlPO 4-8 shows that the true unit cell of the room-temperature polymorph has a doubled c-axis due to a condensed RUM mode. Structured diffuse scattering is also observed which can be related to the thermal excitation of RUM modes. In the case of AlPO 4-tridymite, a complex F1 triclinic polymorph is observed and related to soft RUM modes while, in the case of AlPO 4-16, a soft q= 0 RUM mode is shown to be responsible for an observed phase transition in the case of the all SiO 2 analogue of AlPO 4-16. A large number of additional zero-frequency RUM modes also exist in the case of AlPO 4-16.