Novel hydrides in Mg-TM systems synthesized by high pressure (TM = Zr, Nb, Hf and Ta)

High-pressure synthesis has been widely used for exploration of novel materials. In this study, new Mg-based ternary hydrides in Mg-TM-H systems (TM = Zr, Nb, Hf and Ta) have been prepared under a pressure of 5 GPa by using a cubic-anvil-type apparatus. In TM = Zr, Nb and Hf, MgTM₂H_y with a monoclinic-type structure were synthesized at 923 K under 5 GPa. Lattice parameters of MgTM₂H_y were a = 5.816 Å, b = 3.3569 Å, c = 8.519 Å, β = 103.06° (for TM = Zr); a = 5.70 Å, b = 3.2888 Å, c = 7.927 Å, β = 103.06° (for TM = Nb); a = 5.782 Å, b = 3.317 Å, c = 8.507 Å, β = 103.06° (for TM = Hf). Results from fusion extraction analysis for MgTM₂H_y (TM = Zr, Nb and Hf) showed their hydrogen contents, giving chemical formula of MgTM₂H_5.9. In TM = Nb and Ta, ternary hydrides with an FCC-type structure were synthesized under 5 GPa. These FCC hydrides had lattice parameters of a = 9.5526 Å (TM = Nb) and 9.5376 Å (TM = Ta). For TM = Nb, hydrogen content was determined from the fusion extraction analysis to be 5.8 mass%, corresponding to Mg_6.2NbH₁₄. Among the newly found hydrides, Mg_6.2NbH₁₄ showed the lowest dehydrogenation temperature of 595 K with endothermic reaction. Crystal structures and thermal stabilities of these hydrides were also investigated.