Electronic structure, spin state, and magnetism of the square-lattice Mott insulator La₂Co₂Se₂O₃ from first principles

Electronic and magnetic structures of the recently synthesized cobalt oxyselenide La₂Co₂Se₂O₃ (structurally similar to the superconducting iron pnictides) are studied through density-functional calculations. The obtained results show that this material is a Mott insulator and it has a very stable Co²⁺ high-spin ground state with a t_2g-like orbital ordering, which is substantiated by the calculated crystal-field excitation energies. The square lattice of the Co²⁺ spins is found to have a strong antiferromagnetic (a weak ferromagnetic) coupling for the second-nearest neighbors via O (Se₂) and an intermediate antiferromagnetic one for the first-nearest neighbors with the strength ratio about 10:1:3. The present results account for the available experimental data and the prediction of a planar frustrated (2×2) antiferromagnetic structure would motivate a new experiment.