Magnetic and structural transitions in La 1 − x A x CoO 3 ( A = Ca , Sr, and Ba)

We report thermal-expansion, lattice-constant, and specific-heat data of the series ${\text{La}}_{1\ensuremath{-}x}{A}_{x}{\text{CoO}}_{3}$ for $0\ensuremath{\le}x\ensuremath{\le}0.30$ with $A=\text{Ca}$, Sr, and Ba. For the undoped compound ${\text{LaCoO}}_{3}$, the thermal-expansion coefficient $\ensuremath{\alpha}(T)$ exhibits a pronounced maximum around $T=50\text{ }\text{K}$ caused by a temperature-driven spin-state transition from a low-spin state of the ${\text{Co}}^{3+}$ ions at low temperatures toward a higher spin state at higher temperatures. The partial substitution of the ${\text{La}}^{3+}$ ions by divalent ${\text{Ca}}^{2+}$, ${\text{Sr}}^{2+}$, or ${\text{Ba}}^{2+}$ ions causes drastic changes in the macroscopic properties of ${\text{LaCoO}}_{3}$. The large maximum in $\ensuremath{\alpha}(T)$ is suppressed and completely vanishes for $x\ensuremath{\gtrsim}0.125$. For $A=\text{Ca}$ three different anomalies develop in $\ensuremath{\alpha}(T)$ with further increasing $x$, which are visible in specific-heat data as well. Together with temperature-dependent x-ray data, we identify several phase transitions as a function of the doping concentration $x$ and temperature. From these data we propose an extended phase diagram for ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{CoO}}_{3}$.