Atomic and magnetic structures of (CuCl)LaNb2O7 and (CuBr)LaNb2O7: Density functional calculations

The atomic and magnetic structures of (CuX)LaNb2O7 (X=Cl and Br) are investigated using the density functional calculations. Among several dozens of examined structures, an orthorhombic distorted 2 x 2 structure in which the displacement pattern of X halogens resembles the model conjectured previously based on the empirical information is identified as the most stable one. The displacements of X halogens, together with those of Cu ions, result in the formation of X-Cu-X-Cu-X zigzag chains in the two materials. Detailed analyses of the atomic structures predict that (Cu X)LaNb2O7 crystallizes in the space group Pbam. The nearest-neighbor interactions within the zigzag chains are determined to be antiferromagnetic (AFM) for (CuCl)LaNb2O7 but ferromagnetic (FM) for (CuBr)LaNb2O7. On the other hand, the first two neighboring interactions between the Cu cations from adjacent chains are found to be AFM and FM, respectively, for both compounds. The magnitudes of all these in-plane exchange couplings in (CuBr)LaNb2O7 are evaluated to be about three times those in (CuCl)LaNb2O7. In addition, a sizable AFM interplane interaction is found between the Cu ions separated by two NbO6 octahedra. The fourth-neighbor interactions are also discussed. The present study strongly suggests the necessity to go beyond the square J(1)-J(2) model in order to correctly account for the magnetic property of (CuX)LaNb2O7.