Structural distortion and frustrated magnetic interactions in the layered copper oxychloride (CuCl)LaNb2O7

We present a computational study of the layered copper oxychloride (CuCl)LaNb2O7 that has been recently proposed as a spin-1/2-frustrated square lattice compound. Our results evidence an orbitally degenerate ground state for the reported tetragonal crystal structure and reveal a Jahn-Teller-type structural distortion. This distortion heavily changes the local environment of copper-CuO2Cl2 plaquettes are formed instead of CuO2Cl4 octahedra-and restores the single-orbital scenario typical for copper oxides and oxyhalides. The calculated distortion is consistent with the available diffraction data and the experimental results on the electric field gradients for the Cu and Cl sites. The band structure suggests a complex three-dimensional spin model with the interactions up to the fourth neighbors. Despite the layered structure of (CuCl)LaNb2O7, the spin system has pronounced one-dimensional features. Yet, sizable interchain interactions lead to the strong frustration and likely cause the spin-gap behavior. Computational estimates of individual exchange couplings are in qualitative agreement with the experimental data.