Hydrogen-assisted phase transition in a trihydride MgNi2H3 synthesized at high H-2 pressures: Thermodynamics, crystallographic and electronic structures

MgNi2 intermetallic was synthesized by powder metallurgy and crystallizes with a Laves-type C36 structure (space group P6(3)/mmc (No. 194); a = 4.826; c = 15.832 angstrom). At 300 degrees C during interaction with hydrogen (deuterium) gas compressed to 2.8-7.4 GPa, a trihydride MgNi2H(D)(3.2) was synthesized. The trihydride remained metastable at ambient conditions allowing its structure, stability and magnetic properties to be studied. The formation of MgNi2H3.2 is associated with a complete rebuilding of the initial hexagonal structure into the orthorhombic distorted MoSi2-type sublattice (space group Fmmm (No. 69); a = 4.55; b = 4.69; c = 8.80 angstrom). Neutron diffraction of the MgNi2D3.2 demonstrated that D atoms fill sites having octahedral Mg4Ni2 (D1/4b) and planar Ni2 (D2/8/) coordination. Within the framework of the density functional theory, density of states (DOS) calculations showed the formation of a structure around -10 to -6 eV caused by the chemical bonds of hydrogen and its is states mainly via interaction with the 3d states of Ni. Analysis of the electronic structure revealed a charge transfer from Mg to Ni, and to the H atoms. The calculated enthalpy of formation of MgNi2H3 is about -30 kJ/mol-H-2, which is consistent with the stability of the hydride at normal conditions. The initial sample contained a small amount of a secondary MgNi3 intermetallic, which has been formed during the equilibrium interaction of magnesium and nickel at 800 degrees C. Thus this compound should be included in the phase diagram of the Mg-Ni system. MgNi3 decomposes under high-temperature/ high-pressure hydrogenation conditions and forms nickel monohydride. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.