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-Tc material with an estimated Tc 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 Tc ranging from 117 to 168 K. By combining the method of using two coupling constants λopt and λac, and two characteristic frequencies (optical and acoustic) with first-principle calculations, we found that the high values of Tc are mainly caused by the presence of high frequency optical modes, but the acoustic modes also play a noticeable role.