The first charge-separated iridide
(Ir<sup>3–</sup>) in
an extended solid was identified at elevated pressure when combined
with potassium. According to an unbiased structure searching method
that combines first-principles calculations with particle swarm optimization
algorithms, K<sub>3</sub>Ir in the Cu<sub>3</sub>Ti-type structure
shows a favorable formation enthalpy (Δ<i>H</i>) compared
to the elements and is dynamically stable above 10 GPa. This novel
semiconductor (<i>E</i><sub>g</sub> ≈ 1.6 eV) has
sufficient orbital separation to allow complete charge transfer from
K to Ir, and Bader charge analysis supports the formation of a formally
anionic Ir<sup>3–</sup>. Further, electron doping of K<sub>3</sub>Ir through Pt substitution makes the system metallic, and
electron–phonon coupling calculations indicate that K<sub>3</sub>(Ir<sub>0.875</sub>Pt<sub>0.125</sub>) falls in the strong-coupling
regime, with a predicted superconducting transition temperature (<i>T</i><sub>c</sub>) of ∼27 K at 20 GPa. These results
suggest that systems containing elements isoelectronic with classical
BCS superconductors such as mercury might have an increased probability
of exhibiting a superconducting transition
