Why Mg${}_2$IrH${}_6$ is predicted to be a high temperature superconductor, but Ca${}_2$IrH${}_6$ is not

Abstract

The X${}_2$MH${}_6$ family, consisting of an electropositive cation X and a main group metal M octahedrally coordinated by hydrogen, has been predicted to hold promise for high-temperature conventional superconductivity. Herein, we analyze the electronic structure of two members of this family, Mg${}_2$IrH${}_6$ and Ca${}_2$IrH${}_6$, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg${}_2$IrH${}_6$ the vibrations of the IrH${}_6^{4-}$ anions are key for the superconducting mechanism, and they induce coupling in the $e_g^{\ast }$ set, which are antibonding between the H 1$s$ and the Ir $d_{x^2-y^2}$ or $d_{z^2}$ orbitals. Because calcium possesses low-lying d-orbitals, $e_g^{\ast }\to$ Ca $d$ back-donation is preferred, quenching the superconductivity. Our analysis explains why high critical temperatures were only predicted for second or third row X metal atoms, and may hold implications for superconductivity in other systems where the antibonding anionic states are filled.