Superconductive "sodalite"-like clathrate calcium hydride at high pressures

TL;DR

This paper predicts a high-pressure calcium hydride structure with sodalite-like cages that exhibits superconductivity at temperatures up to 235 K, the highest among hydrides studied so far.

Contribution

It introduces a novel CaH6 structure with sodalite cages stabilized above 150 GPa, revealing a new mechanism for high-temperature superconductivity in hydrides.

Findings

CaH6 forms a sodalite-like cage structure at high pressure.

Superconductivity with Tc of 220-235 K is predicted for CaH6.

The structure's stability is due to electron transfer and Jahn-Teller effects.

Abstract

Hydrogen-rich compounds hold promise as high-temperature superconductors under high pressures. Recent theoretical hydride structures on achieving high-pressure superconductivity are composed mainly of H2 fragments. Through a systematic investigation of Ca hydrides with different hydrogen contents using particle-swam optimization structural search, we show that in the stoichiometry CaH6 a body-centred cubic structure with hydrogen that forms unusual "sodalite" cages containing enclathrated Ca stabilizes above pressure 150 GPa. The stability of this structure is derived from the acceptance by two H2 of electrons donated by Ca forming a "H4" unit as the building block in the construction of the 3-dimensional sodalite cage. This unique structure has a partial occupation of the degenerated orbitals at the zone centre. The resultant dynamic Jahn-Teller effect helps to enhance electron-phonon coupling and leads to superconductivity of CaH6. A superconducting critical temperature (Tc) of 220-235 K at 150 GPa obtained from the solution of the Eliashberg equations is the highest among all hydrides studied thus far.