Local metal and deuterium ordering in the deuterated ZrTiNi C14 Laves phase

Local structure of the hexagonal C14 Laves phase in an annealed approximately equiatomic ZrTiNi alloy was studied before and after deuteration using neutron total scattering. Rietveld refinements of the P63/mmc AB2 model demonstrate that the A-sites are shared by Zr and Ti, whereas the two non-equivalent B-sites are occupied by a mixture of Ti and Ni. Reverse Monte Carlo refinements using a joint fit of the neutron total scattering data in real and reciprocal spaces revealed significant short-range ordering of Ti and Ni on the B-sites. The refined Ti-Ni nearest-neighbor distances are appreciably shorter than the corresponding Ti-Ti and Ni-Ni distances. Nevertheless, the differences in the effective sizes of chemically distinct tetrahedral interstices (i.e. [Zr2Ti2], [Zr2TiNi], [Zr2Ni2], etc.), which are important for hydrogen absorption, remain rather small. Rietveld refinements of the structure of the deuterated sample TiZrNiD2.14 confirmed that deuterium occupies various [A2B2] tetrahedral interstices. RMC refinements suggested a strong preferential occupation of deuterium in the [Zr2Ti2] and [Zr2TiNi] tetrahedra whereas the [Zr2Ni2] sites remained nearly empty. These results support earlier models which predicted preferential occupancy by hydrogen in sites coordinated by metals that form the most stable binary hydrides. The deuterium atoms in the [Zr2TiNi] tetrahedra are displaced toward Ni. X-ray absorption near-edge structure measured for Ti, Zr, and Ni K-edges demonstrated that deuteration is accompanied by the reduction in the density of unoccupied electronic states (just above the Fermi level) associated with Ti and Zr.