Pressure-Induced Phase Transition and Superconducting Properties of LiPdH$_2$: A First-Principles Study

Abstract

We present a detailed first-principles investigation of the superconducting hydride LiPdH$_2$, focusing on the competition between its tetragonal $P4/mmm$ and rhombohedral $R\bar{3}m$ phases. Our results reveal a pressure-induced phase transition near 220 GPa, where $R\bar{3}m$ overtakes $P4/mmm$ in thermodynamic stability. The $R\bar{3}m$ phase is predicted to be superconducting, with $T_{\rm c}$ values of up to 46 K in the harmonic approximation and 28 K at the anharmonic level. In contrast, although the $P4/mmm$ phase is stable up to 200 GPa, it maintains a more rigid electronic character with weaker hydrogen involvement, resulting in lower superconducting performance. Ionic quantum and anharmonic effects play a decisive role in stabilizing $P4/mmm$ and shaping its superconducting properties, whereas these effects diminish rapidly with pressure. These findings highlight the interplay between structural stability, hydrogen orbital activity, and quantum effect in optimizing the superconductivity of hydrogen-based superconductors.