On the determination of the Debye-Waller factor and structure factors of NiAl by X-ray powder diffraction

Abstract

A conventional powder X-ray diffraction experiment was performed on close-to stoichiometric β-NiAl (B2 structure) using a Cu target. From the integrated intensity data the lattice parameters and room temperature Debye-Waller factors of β-NiAl were determined. During the analysis of the integrated intensities, great care was taken to ensure proper treatment of extinction which was found to depend on the mosaic crystal size rather than the powder particle size. Other factors such as dispersion, thermal diffuse scattering and preferred orientation were found to have less of an effect on the integrated intensity analysis. As a result of this improved analytical procedure, Debye-Waller factors for β-NiAl were determined which are in close agreement with the accurate values obtained by single crystal X-ray and electron diffraction. This is in contrast with previous powder X-ray diffraction work on β-NiAl, where poor values of the Debye-Waller factor were usually obtained due to the failure to correct for extinction. The effects of bonding in the solid state were also examined and it was found that the use of accurate crystal structure factors for β-NiAl in the analysis as opposed to those calculated from free atom scattering factors had no significant effect on the experimental Debye-Waller factor values.