Phase equilibria and crystal chemistry of the CaO–1/2 Nd{sub 2}O{sub 3}–CoO{sub z} system at 885 °C in air

The phase diagram of the CaO–1/2 Nd{sub 2}O{sub 3}–CoO{sub z} system at 885 °C in air has been determined. The system consists of two calcium cobaltate compounds that have promising thermoelectric properties, namely, the 2D thermoelectric oxide solid solution, (Ca{sub 3−x}Nd{sub x})Co{sub 4}O{sub 9−z} (0≤x≤0.5), which has a misfit layered structure, and Ca{sub 3}Co{sub 2}O{sub 6} which consists of 1D chains of alternating CoO{sub 6} trigonal prisms and CoO{sub 6} octahedra. Ca{sub 3}Co{sub 2}O{sub 6} was found to be a point compound without the substitution of Nd on the Ca site. The reported Nd{sub 2}CoO{sub 4} phase was not observed at 885 °C. A ternary (Ca{sub 1−x}Nd{sub 1+x})CoO{sub 4−z} (x=0) phase, or (CaNdCo)O{sub 4−z}, was found to be stable at this temperature. A solid solution region of distorted perovskite (Nd{sub 1−x}Ca{sub x})CoO{sub 3−z} (0≤x≤0.25, space group Pnma) was established. In the peripheral binary systems, while a solid solution region was identified for (Nd{sub 1−x}Ca{sub x}){sub 2}O{sub 3−z} (0≤x≤0.2), Nd was not found to substitute in the Ca site of CaO. Six solid solution tie-line regions and six three-phase regions were determined in the CaO–Nd{sub 2}O{sub 3}–CoO{sub z} system in air. - Graphical abstract: Phase diagram of the 1/2 Nd{sub 2}O{sub 3}–CaO–CoO{sub x} system at 885 °C, showing the limits of various solid solutions, and the tie-line relationships of various phases. - Highlights: • Phase diagram of the CaO–1/2 Nd{sub 2}O{sub 3}–CoO{sub z} system constructed. • System consists of thermoelectric oxide (Ca{sub 3−x}Nd{sub x})Co{sub 4}O{sub 9−z} (0≤x≤0.5). • Structures of (Nd{sub 1−x}Ca{sub x})CoO{sub 3−z} and (CaNdCo)O{sub 4−z} determined.