Crystal chemistry and phase equilibria of the CaO-½Eu2O3-CoOz system at 885 °C

•Phase diagram of the Ca-Eu-Co-O system at 885 °C investigated.•Solid solution limit of thermoelectric oxide, (Ca, Eu)3Co4O9, determined.•Phase relationships for understanding properties of thermoelectric Ca-Eu-Co-O obtained.•Structure of distorted perovskite, (Eu1−xCax)CoO3−z, analyzed.

The CaO-½Eu2O3-CoOz system prepared at 885 °C in air consists of two calcium cobaltate compounds, namely, the 2D thermoelectric oxide solid solution, (Ca3−xEux)Co4O9−z (0 ≤ x ≤ 0.5) which has a misfit layered structure, and the 1D Ca3Co2O6 compound which consists of chains of alternating CoO6 trigonal prisms and CoO6 octahedra. Ca3Co2O6 was found to be a point compound without the substitution of Eu on the Ca site when prepared at 885 °C. A solid solution region of distorted perovskite, (Eu1−xCax)CoO3−z (0 ≤ x ≤ 0.22, space group Pnma) was established. The (Eu0.91(1)Ca0.09(1))CoO3−z perovskite member has a distorted structure with tilt angles θ (17.37°), ϕ (8.20°), and ω (19.16°) which represent rotations of an octahedron about the pseudo-cubic perovskite [110]p, [001]p and [111]p axes. The reported Eu2CoO4 phase was not observed at 885 °C, but a ternary Ca-doped oxide, (Eu1+xCa1−x)CoO4−z (Bmab) where 0 ≤ x ≤ 0.10 was found to be stable at this temperature. In the peripheral binary systems, Eu was not present in the Ca site of CaO, while a small solid solution region was identified for (Eu1−xCax)O(3−z)/2 (0 ≤ x ≤ 0.05). Seven solid solution tie-line regions and six three-phase regions were determined in the CaO-½Eu2O3-CoOz system in air.

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