Mossbauer studies of the high-spin-low-spin equilibria and the localized-collective electron transition in LaCoO₃

Abstract

Mossbauer studies combined with magnetic-susceptibility data on well-characterized LaCoO₃ in the 4.2-1200-K region show that cobalt ions exist predominantly in the low-spin Co^III state at low temperatures which transform partially to high-spin Co³⁺ ions up to 200 K. Above 200 K, Co³⁺ and Co^III ion pairs transform to Co^II and Co⁴⁺ pairs. At high temperatures, the population of Co³⁺ decreases significantly and completely disappears at the localized-electron-collective-electron transition temperature at 1210 K. The variations of the lamb-mossbauer 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 LaCoO₃. All these changes are reflected in the transport properties of LaCoO₃. There appears to be little doubt that the first-order localized-electron-collective-electron transition in LaCoO₃ is caused essentially by the change in entropy of the d electrons.