Thermal properties of rare earth cobalt oxides and of La$_{1-x}$Gd$_{x}$CoO$_3$ solid solutions

Powder X-ray diffraction data for the crystal structure, phase composition, and molar specific heat for La$_{1-x}$Gd$_{x}$CoO$_3$ cobaltites in the temperature range of $300$–$1000$ K have been analyzed. The behavior of the volume thermal expansion coefficient in cobaltites with isovalent doping in the temperature range of $100$–$1000$ K is studied. It is found that the $\beta(T)$ curve exhibits two peaks at some doping levels. The rate of the change in the occupation number for the high-spin state of cobalt ions is calculated for the compounds under study taking into account the spin-orbit interaction. With the Birch–Murnaghan equation of state, it is demonstrated that the low-temperature peak in the thermal expansion shifts with the growth of the pressure toward higher temperatures and at pressure $P\sim 7$ GPa coincides with the second peak. The similarity in the behavior of the thermal expansion coefficient in the La$_{1-x}$Gd$_{x}$CoO$_3$ compounds with the isovalent substitution and the undoped LnCoO$_3$ compound (Ln is a lanthanide) is considered. For the whole series of rare earth cobalt oxides, the nature of two specific features in the temperature dependence of the specific heat and thermal expansion is revealed and their relation to the occupation number for the high-spin state of cobalt ions and to the insulator-metal transition is established.