Superconducting phases of YH9 under pressure

TL;DR

This study investigates the crystal structure and superconducting properties of YH9 under pressure, revealing a distorted Pnma phase that explains its positive pressure dependence of Tc, aligning with experimental observations.

Contribution

The paper provides the first detailed structural analysis of YH9 at various pressures, linking structural distortion to its superconducting behavior and pressure dependence of Tc.

Findings

Identified a Pnma distorted cage structure of YH9 at pressures below 220 GPa.

Demonstrated the Pnma phase's positive pressure dependence of Tc matches experimental data.

Showed structural distortion impacts lattice vibrations and electron-phonon coupling, influencing superconductivity.

Abstract

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.