Phase equilibria and crystal chemistry of the CaO–½Sm2O3–CoOz system at 885 °C in air

• Phase diagram of the Sm–Ca–Co–O system at 885 °C studied.
• Solid solution limit of misfit layered (Ca, Sm)3Co4O9 phase determined.
• Solid solution (Sm1−xCax)2O3−z (0 ≤ x ≤ 0.075) limit obtained.
• Structure of (CaxSm1−x)CoO3−z and (Ca1−xSm1+x)CoO4−z determined.

The CaO–½Sm2O3–CoOz system prepared at 885 °C in air consists of two calcium cobaltate compounds, namely, the 2D thermoelectric oxide solid solution, (Ca3−xSmx)Co4O9−z (0 ≤ x ≤ 0.5) which has a misfit layered structure, and the 1D Ca3Co2O6 which consists of chains of alternating CoO6 trigonal prisms and CoO6 octahedra. Ca3Co2O6 was found to be a point compound without the substitution of Sm on the Ca site. A solid solution region of distorted perovskite, (Sm1−xCax)CoO3−z (0 ≤ x ≤ 0.22, space group Pnma) was established. The reported Sm2CoO4 phase was not observed at 885 °C, but a ternary Ca-doped oxide, (Sm1+xCa1−x)CoO4−z (Bmab) where 0 < x ≤ 0.15 was found to be stable at this temperature. In the peripheral binary systems, Sm was not present in the Ca site of CaO, while a small solid solution region was identified for (Sm1−xCax)O(3−z)/2 (0 ≤ x ≤ 0.075). Ten solid solution tie-line regions and six three-phase regions were determined in the CaO–½Sm2O3–CoOz system in air.

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