A coupled electron diffraction and rigid unit mode (RUM) study of the crystal chemistry of some zeotypic AlPO4 compounds
Date
2005
Authors
Withers, Raymond
Liu, Yun
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Publisher
Academic Press
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 AlPO4-8, AlPO4-16 and AlPO4-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±12c* satellite reflections in addition to the Bragg reflections (G) of the underlying Cmc21 parent structure in the case of AlPO4-8 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 AlPO4-16, a soft q=0 RUM mode is shown to be responsible for an observed phase transition in the case of the all SiO2 analogue of AlPO4-16. A large number of additional zero-frequency RUM modes also exist in the case of AlPO 4-16.
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Keywords: Crystal atomic structure; Crystal lattices; Electron diffraction; Electron reflection; Heat treatment; Silica; Characteristic diffuse distributions; Correlated tetrahedral rotation; Dynamical disorder; Microporous aluminophosphates; Aluminum compounds Characteristic diffuse distributions; Correlated tetrahedral rotations; Dynamical disorder; Microporous aluminophosphates
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Source
Journal of Solid State Chemistry
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Journal article
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2037-12-31