Localized-to-Itinerant Electron Transitions in Rare-Earth Cobaltates

M\"ossbauer studies show that cobalt ions in LaCo${\mathrm{O}}_{3}$ exist predominantly in the low-spin ${\mathrm{Co}}^{\mathrm{III}}$ state at low temperatures and partially transform to the high-spin ${\mathrm{Co}}^{3+}$ state up to 200 K. Above 200 K, ${\mathrm{Co}}^{\mathrm{II}}$ and ${\mathrm{Co}}^{4+}$ ions are formed by the transfer of $d$ electrons from ${\mathrm{Co}}^{3+}$ to ${\mathrm{Co}}^{\mathrm{III}}$; ${\mathrm{Co}}^{3+}$ ions completely disappear at the first-order localized-to-itinerant electron transition temperature.