Anomalies of the electronic structure and physical properties of rare-earth cobaltites near spin crossover

The features of the characteristics of LnCoO$_3$ cobaltites, where Ln is a rare-earth element, are discussed. Both experiment and theory demonstrate that their essentials are related to the low-spin ground state of cobalt ions. The thermally induced occupation of the excited high-spin state gives rise to peaks in the magnetic susceptibility, specific heat, and thermal expansion, as well as to a smooth insulator-metal transition. The analysis is based both on the data from the current literature concerning LaCoO$_3$ and in many aspects on our own studies of GdCoO$_3$ and La$_{1-x}$Gd$_x$CoO$_3$ solid solutions.