High-pressure properties of dense metallic zirconium hydrides studied by ab initio calculations

Zirconium-hydrogen compounds under pressure are investigated by using ab initio density functional calculations. In addition to the high-pressure phases so far predicted, other new metallic phases of ZrH₃, ZrH₄, and ZrH₆ are found above about 170, 110, and 150 GPa, respectively. The estimated superconducting transition temperatures of the ZrH₄ and ZrH₆ phases are as follows: 78 K in the Fddd4pt0exZrH₄ (140 GPa), 47 K in the I4/mmm4pt0exZrH₄ (230 GPa), 55 K in the Cmc2₁4pt0exZrH₆ (160 GPa), 153 K in the P2₁/c4pt0exZrH₆ (295 GPa), and 114 K in the I4/mmm4pt0exZrH₆ (295 GPa). The $I4/mmm$ structure of ZrH₄ is actually the structure similarly predicted for MgH₄ and ScH₄. Also, the $I4/mmm$ structure of ZrH₆ is a close modification of the Im3̄m structure proposed for MgH₆ and ScH₆. The structural resemblances observed in these magnesium, scandium, and zirconium hydrides suggest that diagonally adjacent elements tend to have similar chemical trends, namely, diagonal relationships, which are expected to serve as a helpful guide to searching for new metallic hydrides.