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Total Scattering and Reverse Monte Carlo Modelling of Disordered Crystalline Materials

2002; Springer Nature; Linguagem: Inglês

10.1007/978-1-4615-0613-3_6

1567-830X

Matthew G. Tucker, Martin T. Dove, David A. Keen,

Crystal Structures and Properties

The term disordered crystalline materials may seem a contradiction, since crystalline materials are generally considered as an ordered array of atoms with varying degrees of symmetry. This of course is the correct description of the structure when averaged over all the instantaneous position of the atoms in the material. In a standard Rietveld analysis the variation from these average positions is often accounted for by the use of three-dimensional Gaussian functions, where the widths of these functions are called the atomic displacement parameters and the average atom positions are taken to be the mid points of these distributions. These overall distributions are usually referred to as the temperature factors, since they allows for the effects of thermal motion on the average structure. For most crystalline materials this method describes the overall structure of the material very well. However, if the average structure of the crystalline material can accommodate orientational disorder within these temperature factors, as is often indicated if they are highly anisotropic, then the local structure of the material needs to be determined to obtain a true model of the material. Materials which can be described by coordinated polyhedra, such as silica, can often accommodate rotations of these polyhedra while maintaining the overall average structure. This can lead to disordered crystalline materials and, as will be illustrated later, can be the key to understanding the true form of the high temperature structures of this type of materials.