Why MgIrH Is Predicted to Be a High-Temperature Superconductor, But CaIrH Is Not.

The XMH family, consisting of an electropositive cation X and a main group metal M octahedrally coordinated by hydrogen, have been identified as promising templates for high-temperature conventional superconductivity. Herein, we analyze the electronic structure of two members of this family, MgIrH and CaIrH, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within MgIrH the vibrations of the anions IrH anions are key for the superconducting mechanism, and they induce coupling in the set of orbitals, which are antibonding between the H 1s and the Ir or orbitals. Because calcium possesses low-lying d-orbitals, →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 provide rules for identifying likely high-temperature superconductors in other systems where the antibonding anionic states are filled.