Accuracy in Temperature Factor Determination in Powder Diffractometry

Abstract

The discrepancy between the temperature factors derived from X-ray powder diffraction and those derived from X-ray single crystal or neutron powder diffraction led us to look for additional causes of systematic intensity errors for powders. Integrated intensities of powder diffraction lines were carefully measured for selected materials using a Bragg-Brentano powder diffractometer equipped with an automatic divergence slit and a graphite monochromator. The measured intensities were compared with calculated intensities and significant systematic effects were observed which could not be related to structural sources, but were traced to instrumental sources. Use of an empirical angular dependence correction factor resulted in acceptably low R values. The instrumental factor couples strongly to the temperature factor by virtue of a similar angular dependence. Thus, reliable temperature factors cannot be obtained from commercial powder diffractometers unless the instrumental contributions are first removed.