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 IrH₆ ^4- anions are key for the superconducting mechanism, and they induce coupling in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>e</mml:mi> <mml:mi>g</mml:mi> <mml:mo>*</mml:mo></mml:msubsup> <mml:annotation>${e_g^{\ast} }$</mml:annotation> </mml:semantics> </mml:math> set of orbitals, which are antibonding between the H 1s and the Ir <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>d</mml:mi> <mml:mrow><mml:msup><mml:mi>x</mml:mi> <mml:mn>2</mml:mn></mml:msup> <mml:mo>-</mml:mo> <mml:msup><mml:mi>y</mml:mi> <mml:mn>2</mml:mn></mml:msup> </mml:mrow> </mml:msub> <mml:annotation>${d_{x^2 - y^2 } }$</mml:annotation> </mml:semantics> </mml:math> or <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>d</mml:mi> <mml:msup><mml:mi>z</mml:mi> <mml:mn>2</mml:mn></mml:msup> </mml:msub> <mml:annotation>${d_{z^2 } }$</mml:annotation> </mml:semantics> </mml:math> orbitals. Because calcium possesses low-lying d-orbitals, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>e</mml:mi> <mml:mi>g</mml:mi> <mml:mo>*</mml:mo></mml:msubsup> <mml:annotation>${e_g^{\ast} }$</mml:annotation> </mml:semantics> </mml:math> →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.