Diffuse X-Ray Diffraction and Neutron Inelastic Scattering from Modulated Crystals

Abstract

Idealized crystals consisting of a large number of identical atoms or molecules can be considered as macroscopically homogeneous, if in equilibrium with isotropic surroundings specified by external pressure p and temperature T. In this case, their thermodynamical properties are described by the Gibbs potential G(p, T). While the presence of surfaces cannot be ignored entirely, bulk properties prevail in a large uniform crystal that is signified by translational symmetry and periodic boundary conditions. For a crystal undergoing a structural change, the crystalline phases above and below the transition temperature T _c are primarily so idealized in the first approximation. In the transition region. in contrast, the crystal becomes spatially inhomogeneous, arising from locally violated translational symmetry.