Solid State Sciences 5 (2003) 1095–1104 www.elsevier.com/locate/ssscie Structure of LaCuO 2.66 : an oxidized delafossite compound containing hole-doped kagome planes of Cu 2+ cations Ovidiu Garlea, Céline Darie, Catherine Bougerol, Olivier Isnard, Pierre Bordet ∗ Laboratoire de cristallographie CNRS, associé à l’Université J. Fourier, BP166, 38042 Grenoble cedex, France Received 22 January 2003; received in revised form 5 May 2003; accepted 15 May 2003 Abstract 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 LaCuO 2 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.  2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Delafossite; Crystal structure; Kagome 1. Introduction The delafossite-type compounds with general formula A 3+ B + O 2 have been known and studied for a long time [1]. Their structure can be viewed as the stacking of [A 3+ O 2- 2 ] - ∞ layers made of two closed-packed oxygen planes having all octahedral sites occupied by A 3+ cations, which are con- nected by planes of B + cations arranged as a triangular net- work. Each B + cation is coordinated linearly to two oxy- gen anions from the oxygen planes above and below. De- pending on the mutual orientations of successive layers, var- ious polytypes of the delafossite structure can exist. For ex- ample, LaCuO 2 crystallizes according to the rhombohedral 3R polytype shown in Fig. 1 (R ¯ 3m, a 3R = 3.83 Å, c 3R = 17.1 Å), with the stacking sequence O1AO1–B–O2AO2– B–O3AO3–B–O1... , while YCuO 2 is generally reported to crystallize with the hexagonal 2H polytype structure * Corresponding author. E-mail address: pierre.bordet@grenoble.cnrs.fr (P. Bordet). (P 6 3 /mmc, a 2H ≈ 3.52 Å, c 2H ≈ 11.42 Å), with stacking sequence O1AO1–B–O2AO2–B–O1... , where O1, O2 and O3 denote the three different types of oxygen planes in a closed-packed arrangement. These two sequences imply that a 3R = a 2H and c 3R = 3/2c 2H . Since there is no strong chem- ical bond in the B + planes which would introduce strain be- tween the different layers, the undistorted delafossite struc- ture type exists for a large number of A 3+ /B + cation cou- ples, provided they accept the octahedral and linear coordi- nations, respectively. In 1993, Cava et al. [2] have shown that extra oxygen anions can be inserted into delafossite-type RCuO 2 (R = La, Y) compounds by annealing in oxygen atmosphere. The RCuO 2+δ materials obtained conserved the layered nature of the parent structure, with δ values of the extra oxygen con- tent up to ≈ 0.5 for R = Y, and ≈ 0.66 for R = La. These oxygen stoichiometries correspond to an average valence for the copper cations of 2+ and 2.33+, respectively. For both oxidized compounds, the stacking sequence is of type 2H, with an orthorhombic superstructure unit cell (2a 2H , a 2H √ 3, c 2H ) for YCuO 2.5 , and an hexagonal one (a 2H √ 3, a 2H √ 3, c 2H ) for LaCuO 2.66 . Due to the presence of triangular ar- rays of spin 1/2 Cu 2+ cations with antiferromagnetic inter- 1293-2558/$ – see front matter  2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S1293-2558(03)00145-6