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 ${\mathrm{ZrH}}_{3}$, ${\mathrm{ZrH}}_{4}$, and ${\mathrm{ZrH}}_{6}$ are found above about 170, 110, and 150 GPa, respectively. The estimated superconducting transition temperatures of the ${\mathrm{ZrH}}_{4}$ and ${\mathrm{ZrH}}_{6}$ phases are as follows: 78 K in the $Fddd\phantom{\rule{4pt}{0ex}}{\mathrm{ZrH}}_{4}$ (140 GPa), 47 K in the $I4/mmm\phantom{\rule{4pt}{0ex}}{\mathrm{ZrH}}_{4}$ (230 GPa), 55 K in the $Cmc{2}_{1}\phantom{\rule{4pt}{0ex}}{\mathrm{ZrH}}_{6}$ (160 GPa), 153 K in the $P{2}_{1}/c\phantom{\rule{4pt}{0ex}}{\mathrm{ZrH}}_{6}$ (295 GPa), and 114 K in the $I4/mmm\phantom{\rule{4pt}{0ex}}{\mathrm{ZrH}}_{6}$ (295 GPa). The $I4/mmm$ structure of ${\mathrm{ZrH}}_{4}$ is actually the structure similarly predicted for ${\mathrm{MgH}}_{4}$ and ${\mathrm{ScH}}_{4}$. Also, the $I4/mmm$ structure of ${\mathrm{ZrH}}_{6}$ is a close modification of the $Im\overline{3}m$ structure proposed for ${\mathrm{MgH}}_{6}$ and ${\mathrm{ScH}}_{6}$. 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.