Thermodynamical analysis of spin-state transitions in LaCoO₃: Negative energy of mixing to assist thermal excitation to the high-spin excited state

Magnetic susceptibility and heat capacity due to the spin-state transition in LaCoO₃ were calculated by a molecular-field model in which the energy-level diagram of high-spin state reported by Ropka and Radwanski Phys. Rev. B 67 172401 (2003) is assumed for the excited state, and the energy and entropy of mixing of high-spin Co ions and low-spin Co ions are introduced phenomenologically. The experimental data below 300 K were well reproduced by this model, which proposes that the high-spin excited state can be populated even if the energy of high-spin state is much larger than that of low-spin state, because the negatively large energy of mixing reduces the net excitation energy. The stability of each spin state including the intermediate-spin state is discussed based on the present results and other reports.