Rigid unit modes (RUMs) of distortion, local crystal chemistry and the inherent displacive flexibility of microporous AlPO 4-11

Abstract

Electron diffraction is used to investigate the average structure of microporous AlPO 4-11 as well as the zero-frequency rigid unit mode (RUM) modes of distortion of the ideal AlPO 4-11 tetrahedral framework. Direct experimental evidence (in the form of a highly structured, characteristic diffuse intensity distribution) has been found for the (presumably dynamic) excitation of numerous zero-frequency RUM modes of distortion. The lattice dynamic program CRUSH is used to confirm the existence of zero-frequency RUM modes of distortion with wave-vectors falling on the observed diffuse distribution. The simultaneous (presumably dynamic) excitation of such RUM modes of distortion needs to be taken into account in order for the local crystal chemistry of AlPO 4-11 to be understood.