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 Ir3- 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 Pt2- 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.