Accurate Structure Factor Measurements by Powder Methods

Abstract

Powder diffraction data are normally analysed by Rietveld refinement. In this technique the observed diffraction. pattern is fitted to a calculated pattern by least-squares methods. The accuracy with which structure factors can be determined is dependent on the parameters in the model being an accurate representation of the physical situation. The Rietveld model is derived in the kinematic approximation. Deviations from kinematic behaviour because of extinction, absorption, thermal diffuse scattering, and multiple scattering are then included in the model. As a test of the method the Debye-Waller factor (which is the only unknown component of the structure factor) is determined for magnesium oxide from time-of-Bight neutron data on four specimens of very different grain size.