Single-Electron Bond in Ir–Ir Dimer Stabilized under Pressure

Odd-electron bonds, often considered ephemerally stable in chemical systems, can give rise to exotic physical phenomena in quantum materials. In this study, we present the design and discovery of a new iridate, Ba₃NbIr₂O₉, which hosts a single-electron bond within an Ir-Ir dimer. Ba₃NbIr₂O₉ was synthesized under high-pressure and high-temperature conditions and crystallizes in a hexagonal structure (space group <i>P</i>6₃/<i>mmc</i>). The Ir-Ir (III, IV) dimer contains an unpaired electron, leading to an electronically unstable state with potentially frustrated magnetic interactions. Magnetic susceptibility measurements reveal a paramagnetic ground state with no long-range magnetic order down to 1.8 K, despite a Curie-Weiss temperature of -26.9 K. A sign of short-range magnetic ordering is observed below 5 K. The specific heat measurements down to 1.8 K confirm that no long-range ordering is observed in Ba₃NbIr₂O₉. Electronic structure calculations indicate that the Ir-Ir dimer adopts a homogeneous Ir^3.5+-Ir^3.5+. These results highlight a novel design strategy for quantum spin liquid candidates based on targeted manipulation of chemical bonding environments.