In-situ powder neutron diffraction study on the formation process of LaMg₂NiH₇

The formation process from the intermetallic compound LaMg₂Ni to a complex hydride (deuteride) LaMg₂NiD₇ composed of La³⁺, 2 × Mg²⁺, [NiD₄]⁴⁻, and 3 × D⁻ was investigated by in-situ powder neutron diffraction under deuterium gas pressure at room temperature. Below 0.001 MPa, small amount of deuterium was initially dissolved in the lattice of LaMg₂Ni forming LaMg₂NiD_0.05 and two new hydride phases (LaMg₂NiD_x1 and LaMg₂NiD_x2) were continuously yielded. Furthermore, LaMg₂NiD_4.6 with NiD_1.9 and NiD_3.3 units and interstitial deuterium atoms was formed prior to appearing of LaMg₂NiD₇. From their Bragg peak positions, the deuterium contents x1, and x2 were inferred as 0.05 < x1 < x2 < 4.6. At approximately 0.001 MPa, LaMg₂NiD₇ started forming. Since the intermediate deuterides had similar metal atomic framework of LaMg₂Ni, we revealed that LaMg₂NiD₇ formation proceeded through multiple deuteride phases with maintaining of the metal atomic framework of LaMg₂Ni.