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, I 4 / m - MoH 4, I 4 / m m m - MoH 4, C 2 / m - MoH 6, and I m m m - MoH 6. The superconducting critical temperature of I m m m - MoH 6 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 I 4 / m m m - MoH 4 to I 4 / m - MoH 4 at 280 GPa is driven by the imaginary optical modes and the soft optical modes significantly enhance the superconducting critical temperature.