Effect of annealing temperature on the properties of Co_0.33Mn_0.33Fe_2.33O₄ Compound

The compound Co_0.33Mn_0.33Fe_2.33O₄, synthesized by coprecipitation, crystallizes in a well-defined spinel structure belonging to the Fd 3¯ m space group. Increasing the annealing temperature leads to an improvement in crystallinity, as shown by the increase in average crystallite size from 28 to 32 nm. The study of conductivity in the DC regime reveals a metal–semiconductor transition around 420 K for the sample annealed at 300 °C (Co300). This transition disappears after annealing at 600 °C (Co600), indicating a reduction in active traps at low temperature. This conductivity follows a small polaron hopping mechanism, with a high activation energy increasing from 1.29 to 1.34 eV at high temperature. For the Co600 compound, in the AC regime, the activation energy at high temperature remains comparable to that found in the DC regime, with a strong decrease at low temperature (0.29 → 0.14 eV) as frequency increases, highlighting the role of disorder energy (E_D). Frequency analysis reveals that E_D decreases with frequency, while the hopping energy (E_H) increases, suggesting an increase in the polaron radius. The application of the scaling model combines the conductivity spectra into a master curve, validating the time–temperature superposition principle, although divergences appear at high frequency compared to Summerfield’s theory. Introducing a temperature-dependent scaling parameter significantly improves the coalescence. The high values of the temperature coefficient of resistance (TCR), reaching − 30% (Co300) and − 13.8% (Co600), confirm the bolometric potential of the material. Finally, impedance spectroscopy reveals thermally activated relaxation, with activation energies of 778 meV (Co300), 576 meV (high T), and 103 meV (low T) for Co600. Nyquist diagrams show the contribution of grains, grain boundaries, and electrodes to the overall mechanism. These results open promising perspectives for optimizing the material via dopant introduction, control of iron excess, and thin-film development by spray pyrolysis, aiming at electronic and next-generation sensor applications.