Pressure-Induced Phase Transition and Superconducting Properties of LiPdH₂: A First-Principles Study

We present a detailed first-principles investigation of the superconducting hydride LiPdH₂, focusing on the competition between its tetragonal $P4/mmm$ and rhombohedral R3̄m phases. Our results reveal a pressure-induced phase transition near 220 GPa, where R3̄m overtakes $P4/mmm$ in thermodynamic stability. The R3̄m phase is predicted to be superconducting, with Tc 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.