Cu_2IrO_3: A New Magnetically Frustrated Honeycomb Iridate

摘要

We present the first copper iridium binary metal oxide with the chemical formula Cu_2IrO_3. The material is synthesized from the parent compound Na_2IrO_3 by a topotactic reaction where sodium is exchanged with copper under mild conditions. Cu_2IrO_3 has the same monoclinic space group (C2/c) as Na_2IrO_3 with a layered honeycomb structure. The parent compound Na_2IrO_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_2IrO_3 shows a long-range magnetic order at 15 K and fails to become a true spin liquid, Cu_2IrO_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_2IrO_3 with bond angles closer to 120° compared to Na_2IrO_3. Thus, the weak short-range magnetism combined with the nearly ideal honeycomb structure places Cu_2IrO_3 closer to a Kitaev spin liquid than its predecessors.