Trivalent iridium oxides: Layered triangular lattice iridate K_0.75Na_0.25IrO₂ and Oxyhydroxide IrOOH

Solid oxides with transition-metal ions in unusual oxidation states attract enormous attention due to their electronic, magnetic, and catalytic properties. Yet, no crystalline oxide compounds based on purely trivalent iridium have been characterized to date. Here, we present the synthesis and thorough investigation of the properties of the compounds K_0.75Na_0.25IrO₂ and IrOOH, both containing trivalent iridium on a triangular lattice in layers of [IrO₂]. K_0.75Na_0.25IrO₂ crystallizes in a P2-structure with the space group P6₃/mmc, while the crystal structure of IrOOH adopts the direct maximal subgroup P3m1. The trivalent state of the iridium ion is discussed with regards to the iridium-oxygen bond length from X-ray diffraction, the chemical composition, the electronic and magnetic behavior of the material, and X-ray photoemission spectroscopy. The discovery of a new valence state in ternary crystalline iridium oxides is not only of interest from a fundamental perspective, but also has far-reaching implications for such diverse fields as electrochromism, solid-state magnetism, and especially heterogeneous catalysis.