On the disorder of the Cl atom position in and its probable effect on the magnetic properties of (CuCl)LaNb2O7

The magnetic susceptibility of ( CuCl) LaNb2O7 shows a spin gap despite the Cu2+ ions in each CuClO2 layer forming a square lattice. To account for this observation, we explored implications of the disorder of the Cl-atom position in ( CuCl) LaNb2O7 by considering possible ordered structures of CuCl4O2 octahedra in each CuClO2 layer, by identifying various spin exchange interactions of a CuClO2 layer and by estimating the relative strengths of these interactions in terms of spin dimer analysis. We then probed what kind of spin lattice is required for each CuClO2 layer to have a spin gap on the basis of the classical spin approximation. Our study suggests that the CuCl4O2 octahedra of each CuClO2 layer should be arranged such that the resulting spin lattice does not have uniform chains but ring clusters containing an even number of Cu atoms. Implications of this conclusion were discussed on the basis of the recently reported neutron scattering and magnetization studies of ( CuCl) LaNb2O7.