Role of ZnMn2O4 phase in formation of varistor characteristics in ZnO:Mn ceramics

The samples ZnO:Mn were prepared using the conventional solid-state technique. To dope them with manganese, we used water solutions of MnSO4 and MnCl2. The properties inherent to both types of the obtained ceramics have been compared. It was found that the former demonstrated nonlinear current-voltage characteristics, whereas those of the latter were, in fact, linear. The analysis of EPR, diffuse reflectance and Raman spectra obtained for prepared ceramics allowed concluding that, in the samples doped with MnSO4, formation of Mn-related phase, namely, ZnMn2O4 spinel occurred at ZnO grain boundaries under sintering. It has been ascertained that a thin layer of this substance separates adjacent ZnO grains, which provides appearance of the back-to-back Schottky barriers at grain boundaries and “varistor behavior” of current-voltage characteristics.