  Yttrium superhydrides has attracted much attention due to their multiple
stoichiometries and excellent superconductivities under high pressure.
Especially, YH9 have a Tc of 243 K, which is the second highest Tc among all
binary hydrides. It exhibits a positive pressure dependence of Tc below 200
GPa, contrary to the results of theoretical prediction. In order to explore the
origin of Tc at low pressure, we extensively investigated the crystal structure
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, the pressure dependence of Tc in phase Pnma is in line with the
experimental results. Further calculations show the structural distortion
strongly affect the lattice vibration and electron-phonon coupling, leads to
the Pnma phase exhibiting a positive pressure dependence of Tc.
