High-pressure synthesis of novel compounds in an Mg–Ni system

High-pressure works are attractive techniques to obtain new compounds, such as alkali or alkaline earth metal-based systems. The atomic radius of Mg under GPa pressure is considerably smaller compared with transition metals; as such, it may be preferable to synthesize novel intermetallic compounds and hydrides by using high-pressure techniques. In this study, novel compounds were synthesized in an Mg–Ni system by a high-pressure technique using a cubic-anvil-type apparatus.A novel Mg6Ni intermetallic compound was obtained by exposing a mixture of Mg and Ni to 6 GPa at 900 °C for 2 h. The crystal structure of the compound is a tetragonal F-43m structure with a lattice parameter of a=1.9987(1) nm. This compound decomposed to Mg and Mg2Ni phases at 278 °C with exothermic reaction.As is well known, MgNi2 does not form hydrides under conventional hydrogenation conditions, hence we investigated the reactivity of MgNi2 with highly pressurized hydrogen. It was found that the MgNi2 was able to form MgNi2H3.2 by treatment at 700 °C for 2 h under 5 GPa with a hydrogen source, leading to a hydrogen capacity of 2.23 mass%. This novel hydride was found to be a tetragonal MoSi2-type structure (I4/mmm) with lattice parameters of a=0.327(3) nm and c=0.878(9) nm. The dehydrogenation of this hydride occurred at 187 °C with endothermic reaction, and caused decomposition into C36-type MgNi2. This hydride had solubility of Ni content and its thermal stability decreased with increasing Ni content.