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