The relationship between the crystal structure and magnetic frustration in solid state phases is an exciting area of research in physics and materials chemistry. In 2006 Alexei Kitaev proposed a theoretical model for quantum spin liquids (QSL) based on S = 1/2 Fermions on a honeycomb lattice. The crystal structure of A2IrO3 (A = Li, Na) contains honeycomb layers {Ir2A} containing Ir in the oxidation state 4+ (S = 1/2) octahedrally coordinated with 6 oxygen atoms. Thus, the relevance of QSL to A2IrO3 sparked great interest in these materials. However, both Li2IrO3 and Na2IrO3 develop long-range antiferromagnetic ordering (AFM) at 15 K and fail to become true spin liquids. Nearly ten years of research in solid state chemistry and physics was directed to deepen our understanding of physical properties of lithium and sodium iridate. So far, the chemistry of honeycomb iridates has been limited to the homo- and hetero-valent substitutions for the Ir or A sites. All attempts to synthesize a new A2IrO3 have failed due to the large size of K, Rb and Cs. We demonstrate that it is possible to completely exchange Na+ with Cu+ in Na2IrO3 by means of soft chemistry method and synthesize a new honeycomb iridate Cu2IrO3 (Figure 1). Through a complete chemical and structural analysis, we confirm that the solid state ionic exchange reaction can be driven even in the octahedral cages of a honeycomb lattice. Rietveld analysis indicates that the Ir-Ir-Ir bond angles in the honeycomb layers of Cu2IrO3 are closer to ideal 120o and the interlayer spacing is larger than Na2IrO3. Thus, the crystal structure of the new compound is geometrically closer to the Kitaev model than Na2IrO3. Whereas Na2IrO3 shows an AFM order at 15 K, Cu2IrO3 retains a Curie Weiss behavior until 2.7 K where a short-range magnetic order develops. The almost ideal honeycomb structure and the weak short-range magnetism place Cu2IrO3 in the closest proximity to QSL among the family of honeycomb iridates.

Cu2IrO3: Toward the quantum spin liquid phase in honeycomb iridates

255th National Meeting (2018)

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