Mössbauer Studies of the High-Spin-Low-Spin Equilibria and the Localized-Collective Electron Transition in LaCoO3

Mössbauer studies combined with magnetic-susceptibility data on well-characterized LaCo${\mathrm{O}}_3$ in the 4.2-1200-K region show that cobalt ions exist predominantly in the low-spin ${\mathrm{Co}}^{\mathrm{III}}$ state at low temperatures which transform partially to high-spin ${\mathrm{Co}}^{3+}$ ions up to 200 K. Above 200 K, ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{\mathrm{III}}$ ion pairs transform to ${\mathrm{Co}}^{\mathrm{II}}$ and ${\mathrm{Co}}^{4+}$ pairs. At high temperatures, the population of ${\mathrm{Co}}^{3+}$ decreases significantly and completely disappears at the localized-electron-collective-electron transition temperature at 1210 K. The variations of the Lamb-Mössbauer factor and the center shift with temperature provide valuable information on the high-spin-low-spin equilibria, as well as on the nature of the phase transitions and symmetry changes in LaCo${\mathrm{O}}_3$. All these changes are reflected in the transport properties of LaCo${\mathrm{O}}_3$. There appears to be little doubt that the first-order localized-electron-collective-electron transition in LaCo${\mathrm{O}}_3$ is caused essentially by the change in entropy of the $d$ electrons.