Cu2IrO3: a new honeycomb iridate material with an ideal lattice geometry for the Kitaev model*

We present the first binary copper iridium oxide synthesized by a topotactic reaction from sodium iridate and copper chloride under mild conditions. Chemical analysis and crystallographic refinement reveal the composition Cu2IrO3 in a monoclinic C2/c space group with layered honeycomb structure. Magnetization measurements show a magnetic transition at low temperatures T = 2.75 K. The transition is subtle with weak hysteresis in the magnetization and without a clear signature in the heat capacity. The Curie-Weiss analysis reveals that the effective iridium magnetic moment and the exchange coupling strength in Cu2IrO3 are similar to its parent compound Na2IrO3. Resistivity measurements show insulator behavior with variable-range hopping. The frustration index f = 40 in Cu2IrO3 compared to 8 in Na2IrO3 brings copper iridate in closer proximity to a Kitaev spin liquid than its predecessors.