For the rs-Co 0.4 Zn 0.6 O composition a more complicated behavior was observed: the first lines of the wurtzite phase appeared at 773 K, and they coexist with lines of the cubic phase in a rather wide temperature range. At 873 K, additional weak lines appeared which can be correlated with the cubic (Fd-3m) spinel Zn 1-δ Co 2+δ O 4 , apparently due to partial oxidation of the sample by air oxygen. Similar behavior -decomposition into a mixture of three phases -was observed for all compositions with lower ZnO content (0.3 ≤ x ≤ 0.6):