Magnetic and structural transitions in La1-xAxCoO3 (A=Ca, Sr, and Ba)

We report thermal-expansion, lattice-constant, and specific-heat data of the series La(1-x)A(x)CoO(3) for 0 < x < 0.30 with A=Ca, Sr, and Ba. For the undoped compound LaCoO3, the thermal-expansion coefficient alpha(T) exhibits a pronounced maximum around T=50 K caused by a temperature-driven spin-state transition from a low-spin state of the Co3+ ions at low temperatures toward a higher spin state at higher temperatures. The partial substitution of the La3+ ions by divalent Ca2+, Sr2+, or Ba2+ ions causes drastic changes in the macroscopic properties of LaCoO3. The large maximum in alpha(T) is suppressed and completely vanishes for x greater than or similar to 0.125. For A=Ca three different anomalies develop in 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 La1-xCaxCoO3.