Covalence-induced stabilization of an intermediate-spin state and the magnetic susceptibility of LaCoO3

The energies of terms with spins S = 0, 1, 2 have been found using exact diagnoalization of the multielectron Hamiltonian of a multiband pd model for the CoO6 cluster. Co (eg orbital)-O hops, which form the covalent σ bond, are shown to decrease the energy of the state (IS) with an intermediate spin (S = 1) as compared to the energy of the state (LS) with a low spin (S = 0). An analogue of the Tanabe-Sugano diagram that takes into account the covalence of the CoO6 cluster is constructed. The state with S = 1 is shown to be a ground state at certain model parameters. An increase in temperature is established to decrease the crystal field and, thus, favors the transition of the ground state from LS to IS at T = 100 K and the transition of the IS ground state to a state (HS) with a high spin (S = 2) at T = 550 K. The magnetic susceptibility of LaCoO3 is calculated with allowance for the LS, IS, and HS states and for the fact that the HS state exhibits threefold orbital degeneracy of the t2g shell, which results in an effective orbital moment L = 1 and the importance of spin-orbit interaction. The behavior of this magnetic susceptibility agrees well with the experimental x(T) dependence of LaCoO3.