High-Pressure Synthesis of Novel Hydride in Ca-TM Systems

Abstract

Hydrogen storage materials are attracting much attention as media of storing hydrogen. High-pressure synthesis has been widely used for exploration of novel materials. We have reported that many new Mg-based hydrides or alloys have been synthesized by anvil-type apparatus under the pressure of GPa-order. In Mg - TM ( TM = Nb, Ta ) - H systems, it was reported that novel FCC-type hydride which is similar in crystal structure and composition to Mg7TiH13-16 was synthesized under 8 GPa. On the other hand, there is few reports of novel Ca-based hydrides to be synthesized under high pressure. However, the compressibility of calcium is higher than that of magnesium. Thus, there is a tendency for Ca compounds to be synthesized by lower pressure than Mg ones. This study describes the synthesis of new Ca-based hydrides by this high-pressure techniques. In Ca - TM ( TM = Ti, Hf, V, Nb and Ta ) systems, the influence of applied pressure on present phases were investigated. For the composition of CaH2 - 14.3 mol%ZrH2 in Ca - Zr - H system, novel hydride was synthesized at 1073 K for 2 h under 5 GPa. Crystal structure of the novel hydride was found to be FCC-type with a lattice parameter of a = 0.531 nm. In addition, the thermal stability and hydrogen contents of this novel hydride were investigated. In Ca - Hf - H system, the unknown phase was observed in the sample of CaH2 - 12.5 mol%HfH2 prepared 1073 K for 2 h under 5 GPa. This unknown phase is FCC structure with lattice parameter of a = 0.528(2) nm.