Crystal perfection in a noncentrosymmetric alloy. Refinement and test of twinning of the γ-Cu₉Al₄ structure

The crystal structure of Cu₉Al₄ has been refined on the basis of 114 independent structure factors, averaged from a set of 2416 intensity measurements with a diffractometer on a spherical crystal of 0.05 mm diameter. Graphite-monochromatized Cu Kα radiation was used. The final R value was 2.4% when corrections for absorption and extinction had been applied. Anisotropic thermal parameters were introduced and shown to be significant. The occurrence of coherently scattering antiphase domains was found to be less than I%. All equivalent reflections and Friedel pairs were recorded with graphite-monochromatized Mo Kα radiation for ten selected index combinations. Observed and calculated anomalous dispersion effects were found to match closely, indicating that incoherent twinning was also practically non-existent.