Structure of LaCuO_2.66: An oxidized delafossite compound containing hole-doped kagome planes of Cu²⁺ cations

We report the preparation and structural analysis of the LaCuO _2.66 compound. The synthesis is carried out by low temperature (280 °C) oxidation of the LaCuO₂ delafossite. The structure is solved and refined by combined use of electron microscopy, X-ray and neutron powder diffraction. On introduction of oxygen in the copper planes, the 3R rhombohedral stacking of the parent delafossite is reorganized to yield an hexagonal type of stacking. The original copper planes forming a simple triangular lattice are rearranged into a kagome lattice of corner sharing copper cation triangles centered by oxygen anions. This is accompanied by a shortening by ≈ √3/2 of the Cu-Cu distances from ≈ 3.85 Å to ≈ 3.35 Å. These major structural modifications bring about the existence of stacking faults, visible on electron, X-ray and neutron diffraction data, and difficult to take into account in the refinements. Pair distribution function analysis of synchrotron powder diffraction data is used to obtain a direct confirmation of the structure.