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

The discovery of HS and LaH 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 MoH, MoH and MoH compounds were found in the pressure range of 100-300 GPa. The highly hydrogen-rich phase of Cmmm-MoH has a layered structure that contains various forms of hydrogen: H, H and H units. It is a high-T material with an estimated T value in the range of 165-182 K at 250 GPa. The same structures are also found in NbH, TaH, and WH, each material showing T ranging from 117 to 168 K. By combining the method of using two coupling constants λ and λ, and two characteristic frequencies (optical and acoustic) with first-principle calculations, we found that the high values of T are mainly caused by the presence of high frequency optical modes, but the acoustic modes also play a noticeable role.