Hydriding properties of the MgNi-H(D) system with amorphous single phase - investigation from a viewpoint of the short range ordering

The purpose of this work is, first, to get precise information about the thermal stability and the site-occupation of hydrogen (deuterium) in the MgNi-H(D) system with an amorphous single phase, and then to clarify the origin of its hydriding properties from a viewpoint of the short range ordering. The sample was prepared by a mechanical alloying (milling) method, and the structural and hydriding properties were characterized by X-ray and neutron diffraction measurements, and by electrochemical p-c (hydrogen pressure-composition) isotherm measurement. The following results were obtained; (1) The amorphous single phase was prepared by milling for 80 h, after the solid-phase reaction between elemental Ni and nanostructured Mg₂Ni. (2) The p-c isotherm clearly shows that there is a miscibility-gap (plateau), even in the amorphous phase. The hydrogen pressure of the miscibility-gap is located around 3 × 10^-4 MPa at room temperature. (3) Hydrogen (deuterium) occupies the tetrahedral site composed of nearly [2Mg2Ni] in a wide range of hydrogen compositions. (4) The origin of their notable hydriding properties is the short range ordering in the amorphous single phase similar to the CsCl-type cubic structure. The single miscibility-gap in the system is also if importance.