High-pressure synthesis of novel hydrides in Mg-RE-H systems (RE = Y, La, Ce, Pr, Sm, Gd, Tb, Dy)

The high-pressure synthesis of new hydrides of Mg-RE-H systems, where RE = Y, La, Ce, Pr, Sm, Gd, Tb and Dy, were conducted by using a cubic-anvil-type apparatus, and their crystal structure, thermal stabilities and hydrogen contents were investigated. In Mg-Y-H system, newly found MgY₂H _y with a FCC-type structure has been prepared. In MgH₂-x mol% REH (REH = LaH₃, CeH_2.5 and PrH₃), new hydrides with primitive tetragonal structure were synthesized around x = 25-33 under GPa-order high pressures. The lattice constants were a = 0.8193 nm, c = 0.5028 nm, a = 0.8118 nm, c = 0.4979 nm and a = 0.8058 nm, c = 0.4970 nm at x = 25 in Mg-La, Ce and Pr systems, respectively. The hydrogen contents of the novel compounds were 4.1, 3.7 and 3.9 mass% in Mg-La, Ce and Pr systems, respectively, and the chemical formulae were found to correspond to Mg ₃LaH₉, Mg₃CeH_8.1 and Mg ₃PrH₉. The new hydrides decomposed into Mg and rare-earth hydride at about 600 K (Mg₃LaH₉: 614 K, Mg ₃CeH_8.1: 609 K, Mg₃PrH₉: 630 K) with an endothermic reaction. In MgH₂-x mol% hREH (hREH = GdH ₃, TbH₃ and DyH₃), new hydrides with FCC-type structure were synthesized around x = 67.