Phase diagram and crystal chemistry of the La–Ca–Co–O system

The phase diagram of the La–Ca–Co–O system at 885 °C in air has been determined. The system consists of two materials that have interesting thermoelectric properties, namely, the misfit layered thermoelectric oxide solid solution, (Ca,La)3Co4O9, and Ca3Co2O6 which consists of 1D chains of alternating CoO6 trigonal prism and CoO6 octahedra. The reported La2CaO4 and the Ca-doped (La,Ca)2CoO4−z phases were not found at 885 °C. As a result of the absence of these phases, the phase diagram is significantly different from that reported at 1100 °C. Small solid solution regions of (La1−xCax)2O3−z (0 ≤ x ≤ 0.08), (Ca1−xLax)3Co4O9 (0 ≤ x ≤ 0.07), and (La1−xCax)CoO3−z (0 ≤ x ≤ 0.2) were established.

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► Phase diagram of the La–Ca–Co–O system at 885 °C studied.
► Solid solution limit of misfit layered (Ca,La)3Co4O9 phase determined.
► Solid solution (La1−xCax)2O3−x (0 ≤ x ≤ 0.08) limit obtained.