Revisiting Phase Stability and Superconductivity in Ca-H Superhydrides with Anharmonic Effects

TL;DR

This study revisits the phase stability and superconductivity of Ca-H superhydrides, emphasizing the importance of anharmonic effects in accurately predicting stable phases and their superconducting properties at high pressures and temperatures.

Contribution

It provides a revised phase diagram for Ca-H superhydrides considering anharmonic effects, revealing new stable phases and their implications for superconductivity.

Findings

Ca8H46-delta becomes thermodynamically stable at 0 K with anharmonic effects

CaH6 phase stabilizes above 500 K due to anharmonic effects

Insights into structure and superconducting mechanisms of hydrides

Abstract

The prediction of superconductivity above 200 K in CaH$_6$ revolutionized research on hydrogen-rich superconductors, and subsequent experiments have verified this prediction, while unidentified peaks in XRD and the decrease in superconducting temperature upon decompression indicate that unresolved issues remain. In this work, we reconstructed the accurate temperature-pressure phase diagram of the Ca-H system and determined the stability ranges of its candidate superconducting phases by considering anharmonic effects. Our results demonstrate that type-I clathrate Ca$_8$H$_{46-delta}$ structures become thermodynamically stable at 0 K when anharmonic effects are considered. Notably, we found that the previously predicted CaH$_6$ phase achieves stability above 500 K, underscoring the significant role of temperature and anharmonic effects in stabilizing this intriguing high-pressure phase. Our findings offer insights into the structure and superconducting mechanisms of hydrides.