First-principles study on crystal structures and superconductivity of molybdenum hydrides under high pressure

Hydrogen-rich compounds are promising candidates for high temperature superconductors [Duan et al., Natl. Sci. Rev. 4, 121 (2017); Bi et al., arXiv:1806.00163 (2018); Pickard et al., Ann. Rev. Condens. Matter Phys. 11, 57 (2020); Flores-Livas et al., Phys. Rep. 856, 1 (2020)]. Combining first-principles calculation and the swarm intelligence-based structure search method, we investigated structures and superconductivity of molybdenum hydrides under high pressures. Four new phases have been predicted, namely, $I4/m−MoH4$⁠, $I4/mmm−MoH4$⁠, $C2/m−MoH6$⁠, and $Immm−MoH6$⁠. The superconducting critical temperature of $Immm−MoH6$ under 300 GPa was predicted to be 80 K, which is the highest value among hydrides of group six elements in the periodic table. Intriguingly, the phase transition from $I4/mmm−MoH4$ to $I4/m−MoH4$ at 280 GPa is driven by the imaginary optical modes and the soft optical modes significantly enhance the superconducting critical temperature.