K₅Ir: reduced iridium stabilized at high pressure

Alkali binary compounds offer a way to expand our understanding of the periodic table. Specifically, the redox inert nature of alkali cations, even within some intermetallic compounds, enables one to access exotic oxidation states. Iridium presents a test case, as it is the transition metal best positioned to form a trianion, which would be the most highly charged transition metal anion reported. Pressure, known to modulate both reactivity and electronic structure to stabilize exotic phases, is a promising path to this species. Others predicted that reduction of Ir with alkali metals at high pressure may yield phases containing the Ir^3- ion. 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. Based on hybrid functional electronic structure calculations and net atomic charge analysis, K₅Ir is predicted to be a semimetal with a carrier density ∼10²⁰ cm^-3 which features strongly reduced Ir (confirmed by Ir L₃-edge X-ray absorption spectroscopy) and both oxidized and neutral K on different sites. The (negative) net atomic charge of Ir in K₅Ir exceeds those of Pt^2- in Cs₂Pt and Ir(iii-) in putative Na₃[Ir(CO)₃]. First-principles crystal structure prediction indicates that several other K-Ir compounds await discovery.