Crystal chemistry and phase equilibria of the CaO-½Ho2O3-CoOz system at 885 °C in air

Abstract

The phase equilibrium diagram of the CaO-½Ho2O3-CoOz system was determined at 885 °C in air. This diagram offers compatibility relationships in the ternary oxide system that are essential for processing and for the understanding of properties of several thermoelectric phases in the system. Four three-phase regions and three solid solution tie-line regions were determined in the CaO-½Ho2O3-CoOz system. In the CaO-Ho2O3 system, while a small solid solution region was identified for (Ho1-xCax)O(3-z)/2 (0 ≤x ≤ 0.14), Ho was not present in the Ca site of CaO. Neither the reported Ho2CoO4 phase in the Ho2O-CoOz system nor the Ca-doped (Ho1+xCa1-x)CoO4-z phase was present at 885 °C. No solid solution of the distorted perovskite, (Ho1-xCax)CoO3-z, was established at this temperature. The CaO-CoOz system consists of two calcium cobaltate thermoelectric compounds. The 2D thermoelectric oxide, (Ca3-xHox)Co4O9-z (0 ≤x ≤ 0.5), has a misfit layered structure, and the 1D Ca3Co2O6 consists of chains of alternating CoO6 trigonal prisms and CoO6 octahedra. Ca3Co2O6 was found to be a stoichiometric compound. A comparison of the phase diagrams of the CaO-½R2O3-CoOz (R = La, Nd, Eu, and Ho) systems is given.