where k is the Boltzmann constant, T is the absolute temperature. The dependence of the activation energy E a on the composition of the solid solutions, determined by the slope of these linear sections for samples of different compositions, is shown in the inset of Fig. . It is clearly seen that with increasing ZnO content there is a gradual increase in the activation energy from 105 meV for rs-FeO to 144 meV for the rs-Fe 0.5 Zn 0.5 O solid solution.