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

TL;DR

This paper explains why Mg$_2$IrH$_6$ is predicted to be a high-temperature superconductor while Ca$_2$IrH$_6$ is not, based on electronic structure and vibrational coupling differences.

Contribution

It provides a detailed electronic structure analysis revealing the mechanisms behind the differing superconducting potentials of Mg$_2$IrH$_6$ and Ca$_2$IrH$_6$, highlighting the role of d-orbital interactions.

Findings

Mg$_2$IrH$_6$'s superconductivity is driven by IrH$_6^{4-}$ vibrations.

Ca$_2$IrH$_6$'s d-orbital back-donation quenches superconductivity.

High critical temperatures are predicted for second or third row X metals.

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 $e_g^*$ 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, $e_g^* \rightarrow$ 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.