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 FdddZrH4 (140 GPa), 47 K in the I4/mmmZrH4 (230 GPa), 55 K in the Cmc21ZrH6 (160 GPa), 153 K in the P21/cZrH6 (295 GPa), and 114 K in the I4/mmmZrH6 (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 Im3m 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.
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