Determination of the position and occupancy of deuterium in CaNi5 deuterides by neutron diffraction

Abstract

Neutron diffraction measurements were carried out on four kinds of deuteride phases of CaNi5, α (solid solution), α′ (intermediate), β (intermediate), and γ (stable) phases, and the structure parameters, especially the occupation position and occupancy of deuterium atoms in each phase, were determined by refinement of the intensity data with Rietveld analysis. The space groups determined most suitable were P6/mmm, Im2m, Im2m, and P31m, corresponding to the α, α′, β, and γ phase, respectively. Deuterium atoms in the α phase were found to occupy both the octahedral and tetrahedral sites randomly at essentially the same rate. In the α′ phase, occupation of octahedral sites proceeded in preference to tetrahedral sites. Differences in occupation in the number of deuterium atoms arose simultaneously between the same types of site, which is reflected in anisotropic expansion in the basal plane. In the β phase, the occupation of tetrahedral sites is larger than that in the α′ phase, and a difference in occupancy developed more conspicuously between both types of sites. In the γ phase, the occupancies reached their ultimate level, being unity for octahedral sites and 1/2 for tetrahedral sites. The differences in occupancy have disappeared, leading again to isotropic expansion in the basal plane. It was thus revealed that the stabilization of the intermediate deuteride phases of CaNi5 is caused by different occupation numbers of deuterium atoms for sites of the same type, both for the octahedral and the tetrahedral type.