Theoretical study on temperature effect of electronic structure and spin state in LaCoO₃ by using density functional theory

We analyzed the X-ray absorption spectra (XAS) of Co 2p orbital in LaCoO₃ by using density functional theory method to understand the temperature effect on the electronic structures, especially low-spin (LS), intermediate-spin (IS), and high-spin (HS) states of Co^{3 +} ion. We considered the lattice change, thermal excitation of electrons, and displacement of atoms from equilibrium position in LaCoO₃ as a temperature effect in this study. We observed negligible change in the shape of Co 2p XAS of LaCoO₃ for each LS, IS, and HS state associated with the lattice expansion due to temperature increase, while different spectral features for LS, IS, and HS states were observed. The change of spectra observed by in situ XAS measurement of LaCoO₃ at Co L_II- and L_III-edges would be due to the increase in HS and decrease in LS and IS at higher temperature. In addition, the geometry change by thermal displacement of atomic position was analyzed by using first-principles molecular dynamics calculation. Our results indicated that the electronic structure is sensitively changed by the change of Co-O bond distance due to the displacement of atoms from the equilibrium position at studied temperature when the spin state was IS.