Hydrogen-rich scandium Compounds at high pressures

摘要

Scandium hydrides at high pressures have been investigated by using ab initio density functional calculations. Although the stable scandium hydride so far known to have the highest content rate of hydrogen is ScH_3, other more hydrogen-rich compounds are found to be possible at high pressures. These are ScH_4 in the 14/mmm structure above 160 GPa, ScH_6 in the P6_3/mmc structure from 135 to 265 GPa, and ScH_6 in the Im3m structure above 265 GPa. The three phases are all metallic, and the superconducting transition temperatures estimated from the extended McMillan equation are 67 K in the I4/mmm ScH_4 at 195 GPa, 63 K in the P6_3/mmc ScH_6 at 145 GPa, and 130 K in the Im3m ScH_6 at 285 GPa. While the 14/mmm tetrahydride and the Im3m hexahydride were similarly predicted for yttrium (another group-3 element), the P6_3/mmc hexahydride is possible only for scandium. The smaller atomic size of scandium stabilizes the P6_3/mmc structure, and other nearby d-block elements, whose atomic sizes are smaller or comparable, might be likewise capable of forming such poly hydrides.