Superconducting phases of YH₉ under pressure

Yttrium superhydrides have attracted much attention due to their multiple stoichiometries and excellent superconductivity under high pressure. Especially, YH9 have a superconducting critical temperature (T-c) of 243 K, which is only second to LaH10 among all superconductors. It exhibits a positive pressure dependence of T-c below 200 GPa, contrary to the results of theoretical prediction. In order to explore the origin of T-c at low pressure, we extensively investigated the crystal structures of YH9 at different pressure, and found a distorted cage structure with a symmetry of Pnma. This phase has the lowest enthalpy at pressure below 220 GPa, and its x-ray-diffraction patterns is consistent with experimental data. Most importantly, its pressure dependence of T-c is in line with the experimental results indicating that the low T-c at low pressure mainly comes from the low-symmetry Pnma phase. Further calculations show the structural distortion in the Pnma phase strongly affects the lattice vibration and electron-phonon coupling induced a positive pressure dependence of T-c. This work uncovers a remarkable correlation between superconductivity and structural distortion, and provides insight into clarifying the dome-shaped superconducting phase diagram (T-c-P) in yttrium hydrides.