Phase equilibria and crystal chemistry of the CaO–[formula omitted]Nd2O3–CoOz system at 885 °C in air

Abstract

The phase diagram of the CaO–12Nd2O3–CoOz 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, (Ca3−xNdx)Co4O9−z (0≤x≤0.5), which has a misfit layered structure, and Ca3Co2O6 which consists of 1D chains of alternating CoO6 trigonal prisms and CoO6 octahedra. Ca3Co2O6 was found to be a point compound without the substitution of Nd on the Ca site. The reported Nd2CoO4 phase was not observed at 885°C. A ternary (Ca1−xNd1+x)CoO4−z (x=0) phase, or (CaNdCo)O4−z, was found to be stable at this temperature. A solid solution region of distorted perovskite (Nd1−xCax)CoO3−z (0≤x≤0.25, space group Pnma) was established. In the peripheral binary systems, while a solid solution region was identified for (Nd1−xCax)2O3−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–Nd2O3–CoOz system in air.