Abstract
Electron diffraction is used to investigate the unit cell and space group symmetry of the average structure of AlPO 4-5 as well as the zero frequency rigid unit mode (RUM) modes of distortion thereof. Despite recent reports to the contrary, absolutely no evidence could be found to justify the lowering of the average structure space group symmetry of AlPO 4-5 from P6 cc. Direct experimental evidence (in the form of a highly structured, characteristic diffuse intensity distribution) is found, however, for the (presumably dynamic) excitation of numerous zero frequency RUM modes of distortion of the ideal AlPO 4-5 tetrahedral framework structure. The lattice dynamic program CRUSH is used to confirm the existence of these zero frequency RUM modes of distortion as well as to show that they are an intrinsic, predictable property of the AlPO 4-5 tetrahedral framework structure. Six zero frequency RUM modes of distortion are predicted to exist for modulation wave-vectors of the form [hk0] ∗ , 4 for wave-vectors of the form [00l] ∗ and a single RUM for wave-vectors falling on a curved (hyperbolic) distribution. These RUM modes of distortion involve tetrahedral rotation about in-plane rotation axes. Their simultaneous (presumably dynamic) excitation is necessary to understand the apparent linear AlOP angles in average structure refinements.
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