High-pressure synthesis of novel hydride in Mg-Ni-H and Mg-Ni-Cu-H systems

Novel hydrides were synthesized in Mg-Ni-H and Mg-Ni-Cu-H systems by high-pressure technique using a cubic-anvil-type apparatus. Crystal structure and thermal stability of the hydrides were studied. The novel hydride with a composition of MgNi₂H_y was synthesized at 973 K for 2 h under the pressure of over 2 GPa. This hydride could be synthesized from each starting material, such as MgNi₂(C36) and mixture of MgH₂-x mol% Ni (x = 65-70). From fusion analysis, hydrogen content of the sample of MgH₂-67 at.% Ni prepared with hydrogen source at 973 K for 2 h under 5 GPa was estimated to be 2.23 mass% and chemical formula was corresponding to be MgNi₂H_3.2. The novel hydride was found to exhibit the body-centered tetragonal structure (space group I4/mmm, No. 139) with lattice parameters of a = 0.327(3) nm, c = 0.878(9) nm. Moreover, crystal structure of novel hydride was distorted with varying Ni content, then it was found to exhibit C-faced based-centered orthorhombic structure (space group Pmmm, No. 47) with lattice parameters of a = 0.460(1) nm, b = 0.468(1) nm, c = 0.893(1) nm. In Mg-Ni-Cu-H system, crystal structure and thermal stability of Mg(Ni_1-xCu_x)₂H_y (x = 0.05, 0.10, 0.15, 0.20) were investigated. The sample of Mg(Ni_0.95Cu_0.05)₂ prepared with hydrogen source at 973 K for 8 h under 5 GPa was dehydrogenated at 446 K, which is 15 K lower than that of MgNi₂H_3.2. Moreover, Mg(Ni_0.95Cu_0.05)₂ prepared with hydrogen source at 973 K for 2 h under 5 GPa,which did not react hydrogen sufficiently, was decomposed at 409 K.