Wavelength Dependence of Moderate Extinction Effects in an Extended-face Crystal of Cadmium Selenide

Abstract

X-ray intensity measurements have been carried out with an extended-face single crystal of hexagonal CdSe using four different wavelengths. The wavelength dependence of moderate extinction effects (y > 0·7, where y is the secondary extinction factor) was investigated by analysing the Bragg intensities using Zachariasen's (1967) theory, with and without the sin 26 factor (6 being the Bragg angle), which was claimed to have been omitted from the original expression for the diffraction cross section in a perfect crystallite (Becker and Coppens 19740, 1974 b). The resulting Hamilton (1965) R factors for the different extinction models tested do not enable one to make conclusive statements in regard to the 'best' model, but the refined values of the domain radius r and the mosaic-spread parameter g suggest that the crystal is neither of type I (Le. r > Ag, where A is the wavelength) nor of type II (Le. r < Ag). The wavelength dependence of the extinction effects is adequately accounted for in terms of the models tested.