Crystal structure and hydrogenation properties of pseudo-binary Mg6Pd0.5Ni0.5 complex metallic alloy

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 Mg6Pd compound. It crystallizes in F (4) over bar 3m space group with lattice parameter 20.13331(7) angstrom. The refined unit-cell composition is Mg-342(1)Pd29(1)Ni23(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 MgD2, Mg5Pd2 and Mg2NiD4 phases. The Mg2NiD4 phase is observed to crystallize in the orthorhombic LT2 modification for which an averaged crystal structure in the Pcc2 space group is proposed.