TEMPERATURE-INDUCED STRUCTURAL-CHANGES IN BE2ZRD15 STUDIED BY POWDER NEUTRON-DIFFRACTION

Based on Rietveld analysis of powder neutron diffraction data, the deuterium atoms in Be2ZrD1.5 are found at 298 K to occupy trigonal bipyramidal sites. In the AlB2 type structure, two tetrahedral and one trigonal bipyramidal sites are closely located and can only be occupied by one deuterium atom. The coordination found at 298 K may represent an average of deuterium atoms occupying all the three mentioned positions, which is indicated by a large displacement factor. At low temperatures, T<250 K, new reflections appear indicating a superstructure. The few additional reflections did not allow determination of the low temperature structure. However, the Rietveld refinements of the filled up AlB2 type structure in space group P6/mmm, showed significant improvement for the model where deuterium solely occupies tetrahedral sites. The results indicate a continuous transition from a (partly) ordered structure at T=12 K with deuterium in tetrahedral sites, to a disordered structure with deuterium, in average, filling the trigonal bipyramidal sites at 298 K.