Why Mg2IrH6 Is Predicted to Be a High‐Temperature Superconductor, But Ca2IrH6 Is Not.

The X2MH6 family, consisting of an electropositive cation Xn+ 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, Mg2IrH6 and Ca2IrH6, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg2IrH6 the vibrations of the anions IrH64− anions are key for the superconducting mechanism, and they induce coupling in the eg* ${e_g^{\ast} }$ set of orbitals, which are antibonding between the H 1<italic>s</italic> and the Ir dx2-y2 ${d_{x^2 - y^2 } }$ or dz2 ${d_{z^2 } }$ orbitals. Because calcium possesses low‐lying <italic>d</italic>‐orbitals, eg* ${e_g^{\ast} }$ →Ca <italic>d</italic> 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. [ABSTRACT FROM AUTHOR]