K5Ir: Reduced Iridium Stabilized in a High-Pressure Semimetal

Alkali binary compounds offer a way to expand our understanding of the periodic table. Specifically, the redox inert nature of these cations, even within some intermetallic compounds, enables one to access exotic oxidation states. Prior work on semiconducting alkali aurides(I-) and platinides(II-) containing transition metal anions stimulated theoretical predictions of the monatomic iridide(III-) anion by reduction of iridium with alkali metals under pressure. We tested these predictions by reacting a K-rich mixture of K and Ir at 19.5(6) GPa and 493 K. This reaction yields K5Ir, which adopts the rare but simple BaSn5 crystal structure. Hybrid functional electronic structure calculations, net atomic charge analysis, and Ir L3-edge X-ray absorption spectroscopy reveal K5Ir is a semimetal with a carrier density ~10^20 cm^−3 which features anionic Ir and both cationic and neutral K on different sites. While the net atomic charge of Ir in K5Ir falls short of that in hypothetical, semiconducting K3Ir, it exceeds those of Pt(II-) in Cs2Pt and Ir(III-) in [Ir(CO)3]^3−, suggesting an extreme for the distribution of charge in the vicinity of a transition metal. First-principles crystal structure prediction corroborates the thermodynamic stability of K5Ir under the preparatory conditions and indicates that several other K−Ir compounds await discovery.