High-pressure synthesis of the hydrides in Mg-M (M = Mn, Y) systems and the influence of applied pressures during synthesis on present phases and their crystal structures have been studied. In Mg-Mn system, it was found that the crystal structure of Mg3MnHy changed from hexagonal structure (a = 0.47107(4) nm and c = 1.0297(1) nm) to monoclinic structure (a = 0.8819(8) nm, b = 0.4658(4) nm, c = 0.4678(5) nm and β = 105.6(1)°) in a pressure range of 3-3.5 GPa. This crystal structural change was reversible with respect to pressure. The Mg3MnHy synthesized under 5 GPa was stable up to around 620 K. From thermogravimetric and fusion extraction analyses, the hydrogen content was determined as Mg3MnH 5.0-5.6. In Mg-Y system, the high-pressure hydride (MgY 2Hy) with yellowish color was synthesized at 1073 K for 2 h under 3 GPa or higher. This phase exhibited an FCC-type structure with a cell parameter of a = 0.516 nm. Its hydrogen content was determined to be about 3.7 mass%, corresponding to a chemical formula of MgY2H7.8. The hydride was partially dehydrogenated at around 600 K, and the amount of hydrogen partially desorbed was 1.4 mass%. The FCC-type structure was stable even after the partial dehydrogenation.
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