Cu₂IrO₃: A New Magnetically Frustrated Honeycomb Iridate

Abstract

We present the first copper iridium binary metal oxide with the chemical formula Cu 2 IrO 3 . The material is synthesized from the parent compound Na 2 IrO 3 by a topotactic reaction where sodium is exchanged with copper under mild conditions. Cu 2 IrO 3 has the same monoclinic space group ( C 2/ c ) as Na 2 IrO 3 with a layered honeycomb structure. The parent compound Na 2 IrO 3 is proposed to be relevant to the Kitaev spin liquid on the basis of having Ir 4+ with an effective spin of 1/2 on a honeycomb lattice. Remarkably, whereas Na 2 IrO 3 shows a long-range magnetic order at 15 K and fails to become a true spin liquid, Cu 2 IrO 3 remains disordered until 2.7 K, at which point a short-range order develops. Rietveld analysis shows less distortions in the honeycomb structure of Cu 2 IrO 3 with bond angles closer to 120° compared to Na 2 IrO 3 . Thus, the weak short-range magnetism combined with the nearly ideal honeycomb structure places Cu 2 IrO 3 closer to a Kitaev spin liquid than its predecessors.