Effect of synthesis pressure on hydride phases in Mg–M systems (M = Mn, Y)

Abstract

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 b= 105:6(1)8) 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 Mg3MnH5.0–5.6. In Mg–Y system, the high-pressure hydride (MgY2Hy) 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.