Cu₂IrO₃: A New Magnetically Frustrated Honeycomb Iridate

We present the first copper iridium binary metal oxide with the chemical formula Cu₂IrO₃. The material is synthesized from the parent compound Na₂IrO₃ by a topotactic reaction where sodium is exchanged with copper under mild conditions. Cu₂IrO₃ has the same monoclinic space group (C2/c) as Na₂IrO₃ with a layered honeycomb structure. The parent compound Na₂IrO₃ is proposed to be relevant to the Kitaev spin liquid on the basis of having Ir⁴⁺ with an effective spin of 1/2 on a honeycomb lattice. Remarkably, whereas Na₂IrO₃ shows a long-range magnetic order at 15 K and fails to become a true spin liquid, Cu₂IrO₃ remains disordered until 2.7 K, at which point a short-range order develops. Rietveld analysis shows less distortions in the honeycomb structure of Cu₂IrO₃ with bond angles closer to 120° compared to Na₂IrO₃. Thus, the weak short-range magnetism combined with the nearly ideal honeycomb structure places Cu₂IrO₃ closer to a Kitaev spin liquid than its predecessors.