Anisotropic corrections of measured integrated Bragg intensities for thermal diffuse scattering. II

Abstract

The method of evaluation for the contribution of TDS to Bragg reflexions is given on the basis of the general formalism developed in the previous paper [Harada & Sakata, Acta Cryst. (1974), A30, 77-82]. TDS tensor Δβ is expressed by a matrix product as {\tilde{\sigma}}Tσ, where T is the tensor that characterizes the anisotropy of TDS in reciprocal space and σ is the transformation matrix of scattering vector from Cartesian axes to crystallographic reciprocal axes. All the components of T and σ are listed in a table for the nine groups of elastic constants for practical use. It is, however, found that there are only seven matrix forms of Δβ corresponding to the seven crystallographic systems. Two different approximations proposed previously for Nillson's formalism in the estimation of the scan area of Bragg reflexion are shown to be available also for the general formula. The numerical calculations of the TDS correction for an NaC1 single crystal are made with these approximations and they are compared with the experimental measurements by Renninger and with the results of calculations given with other approximations. No substantial difference is seen among the results of calculations and they are in good agreement with experiments.