High-pressure synthesis of novel hydrides in Mg-RE systems and their hydrogen content (RE = Y, La)

New high-pressure hydrides in the MgH₂-REH₃ systems, where RE = Y and La, have been prepared by using a cubic-anvil-type apparatus, and their detailed crystal structure and hydrogen content have been investigated. The high-pressure synthesis was performed at 800 °C under 5 GPa for 2 h. In the Mg-Y system, MgY₂H_7.8 (H/M ≈ 2.6) with an FCC structure (a = 0.51657(2) nm) have been prepared. Based on the hydrogen content and crystallographic consideration, it was suggested that octahedral sites were partially (≈ 60%) occupied by H atoms in addition to tetrahedral sites. For this hydride, H atoms on the tetrahedral sites seem to play a role to hold the FCC unit cell. For MgH₂ - x mol% LaH₃, the new high-pressure hydride was obtained in the range of 25 ≤ x ≤ 33. Mg₃LaH₉ was found to have a primitive tetragonal unit cell (a = 0.8172 nm, c = 0.5015 nm).