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 MgNi₂H_3.2 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.