Crystal structure and hydrogenation properties of pseudo-binary Mg 6Pd 0.5Ni 0.5 complex metallic alloy

Abstract

The crystal structure of the Ni-substituted Mg 6.10(2)Pd 0.52(2)Ni 0.41(2) complex metallic alloy has been determined by X-ray and neutron powder diffraction. The reaction of this compound at 573 K towards deuterium absorption for pressures up to 23 bar has also been studied. The crystal structure of Mg 6.10(2)Pd 0.52(2)Ni 0.41(2) compound was determined in the light of Samson's [Acta Crystallogr. B 28 (1972) 936) and Makongo's (Philos. Mag. 86 (2006) 427] models for the binary Mg 6Pd compound. It crystallizes in F 4 ¯ 3 m space group with lattice parameter 20.13331(7) Å. The refined unit-cell composition is Mg 342(1)Pd 29(1)Ni 23(1) with Z=56. Nickel by palladium substitution is not fully random. Nickel atoms preferentially locate on Pd sites with low coordination number due to steric effects. Deuterium uptake is 9.6 D/f.u. under the given conditions of pressure and temperature. Upon absorption, the intermetallic compound disproportionates into MgD 2, Mg 5Pd 2 and Mg 2NiD 4 phases. The Mg 2NiD 4 phase is observed to crystallize in the orthorhombic LT2 modification for which an averaged crystal structure in the Pcc2 space group is proposed.