Phase Equilibria of the Zinc Oxide–Cobalt Oxide System in Air

Phase equilibria of the zinc oxide–cobalt oxide system were studied by a combination of X ‐ray diffraction and in situ electrical conductivity and thermopower measurements of bulk ceramic specimens up to 1000°C in air. Rietveld refinement of X ‐ray diffraction patterns demonstrated increasing solubility of Co in ZnO with increasing temperature, which is supported by the slight increase in wurtzite ( Zn 1− x Co x O ) cell volume and lattice parameter a versus temperature determined for the phase boundary compositions. Similarly, the solubility of Zn in CoO increased with increasing temperature. In contrast, the spinel phase ( Zn z Co 3− z O 4 ) exhibited retrograde solubility for Zn . Electrical measurements showed that the eutectoid temperature for transformation of rocksalt Co 1−y Zn y O into wurtzite and spinel is 894 ± 3°C, and the upper temperature limit of the stability of the spinel phase is 894°C–898°C for the compositions Co /( Zn + Co ) = 0.82–1. The resulting composition‐temperature phase diagram is presented and compared to the earlier (1955) diagram by Robin.