Thermal diffuse scattering in cubic powder patterns

The one-phonon thermal diffuse scattering in powder patterns of monatomic cubic materials has been investigated using a model that gives the correct frequencies and polarization vectors of the long-wavelength phonons for materials of arbitrary elastic anisotropy. Computer calculations have been made of the intensity distribution of this scattering and of the correction for its inclusion in measured integrated intensities of powder pattern reflections. Elastic anisotropy is found to produce marked differences, despite the powder pattern averaging; and, contrary to the Chipman-Paskin approximation, the integrated intensity correction is found generally not to vary smoothly with (h² + k² + l²) or linearly with scan length even for isotropic materials, as Suortti also noted. A much simpler method for calculating the integrated intensity correction has also been developed, based on a modified Warren model, that gives reasonably accurate values under most conditions even for very anisotropic materials and is several orders of magnitude faster than the primary method.