Temperature-induced spin-state transitions in LaCoO3:: A two-level model

We investigate the low-spin to high-spin transition of cobalt in LaCoO3. The underlying physics of the transition is related to the large totally symmetric relaxation of ligands accompanying the electron conversion from the t(2g) to the e(g) orbitals of Co3+. The electron distribution function, the electronic entropy, the heat capacity, the susceptibility, and the expansion coefficient are determined using a two-level microscopic model. The gradual low-spin to high-spin transition of Co3+ with temperature is calculated, and the results are in good qualitative agreement with known experimental data. In order to explain the observations it is not necessary to invoke an intermediate-spin state.