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-xCo_xO) 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_zCo_3-zO ₄) exhibited retrograde solubility for Zn. Electrical measurements showed that the eutectoid temperature for transformation of rocksalt Co _1-yZn_yO 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.