High-temperature superconductivity in transition metallic hydrides MH₁₁ (M = Mo, W, Nb, and Ta) under high pressure

The discovery of H₃S and LaH₁₀ is an important step towards the development of room temperature superconductors which fuels the enthusiasm for finding promising superconductors among hydrides at high pressure. In the present study, three new and stable stoichiometric MoH₅, MoH₆ and MoH₁₁ compounds were found in the pressure range of 100-300 GPa. The highly hydrogen-rich phase of Cmmm-MoH₁₁ has a layered structure that contains various forms of hydrogen: H, H₂^- and H₃^- units. It is a high-T_c material with an estimated T_c value in the range of 165-182 K at 250 GPa. The same structures are also found in NbH₁₁, TaH₁₁, and WH₁₁, each material showing T_c ranging from 117 to 168 K. By combining the method of using two coupling constants λ_opt and λ_ac, and two characteristic frequencies (optical and acoustic) with first-principle calculations, we found that the high values of T_c are mainly caused by the presence of high frequency optical modes, but the acoustic modes also play a noticeable role.