Anisotropic Strain-like Line Broadening Due to Composition Variations

The line-broadening of a sample of a powder of non-stoichiometric hexagonal ε-iron nitride with the average composition FeN0.433 was analysed by X-ray diffraction. A nearly Gaussian purely strain-like broadening was observed. The broadening is strongly orientation dependent. For each direction of the diffraction vector relative to the crystallographic axes the width of the reflections varies proportional to tanθ, with 2θ as the diffraction angle. The shape of the observed anisotropy could be fitted by a model based on the assumption of small differences between the compositions of the different powder particles. The observed extent of the anisotropy was found to be compatible with the known composition dependence of the hexagonal cell parameters of ε-FeNy. It was shown, that for all crystal systems strain-like line broadening caused by composition variations can be conceived as a special case of the phenomenological description of anisotropic strain broadening often used in Rietveld refinement programmes (P. W. Stephens, J. Appl. Cryst. 1999, 32, 281-289.).