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 ScH3, other more hydrogen-rich compounds are found to be possible at high pressures. These are ScH4 in the I4/mmm structure above 160 GPa, ScH6 in the P63/mmc structure from 135 to 265 GPa, and ScH6 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/mmmScH4 at 195 GPa, 63 K in the P63/mmcScH6 at 145 GPa, and 130 K in the Im3mScH6 at 285 GPa. While the I4/mmm tetrahydride and the Im3m hexahydride were similarly predicted for yttrium (another group-3 element), the P63/mmc hexahydride is possible only for scandium. The smaller atomic size of scandium stabilizes the P63/mmc structure, and other nearby d-block elements, whose atomic sizes are smaller or comparable, might be likewise capable of forming such polyhydrides.
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