Liquid GeBr4. I. A test of the anomalous x-ray scattering method as applied to disordered materials

Abstract

This study tested the accuracy of the anomalous x-ray scattering technique as part of an investigation of the molecular liquid GeBr4. The differential anomalous scattering technique proved to be quite accurate, giving nearest-neighbor coordination distances and numbers correct to 0.02 Å and 4%, respectively, as compared with known values. The high accuracy of the approach evidently depends on a cancellation of systematic errors when subtracting data sets at two nearby photon energies. The straightforward three-wavelength approach to determining the partial-distribution functions is quite ill conditioned and small experimental errors cause large errors in the resulting partial distribution functions. An alternate procedure, suggested by Munro, uses differences to determine the partial distribution functions (PDF’s) and hence takes advantage of the cancellation of systematic errors. This approach yields Br–Br, Ge–Br, and Ge–Ge PDF’s with error levels of approximately 5%–10%, 30%, and 100%, respectively. Singular value decomposition of the weighting matrix can be used to predict the relative accuracies of the three PDF’s before experimentation.