Elucidating the role of chromium substitution at the iron sites on the structural evolution and electrical...

Multifunctional spinel ferrites have attracted considerable attention for temperature-sensing and thermistor applications due to their tunable electrical properties and thermal stability. In this work, CuFe_2-x Cr _x O₄ spinel ferrites (x = 0 and 0.5) were synthesized by the conventional solid-state reaction method in order to investigate the effect of chromium substitution on their structural, microstructural, and electrical properties. X-ray diffraction analysis confirmed the formation of single-phase cubic spinel structures with the Fd3̄m space group. FTIR and XPS investigations further verified the successful incorporation of Cr³⁺ ions into the ferrite lattice. Chromium substitution induced a slight contraction of the lattice parameter from 8.3885 Å to 8.3689 Å, accompanied by modifications in the local bonding environment. SEM observations and density measurements revealed compact microstructures with relatively low porosity. Electrical measurements performed in the 300-600 K temperature range showed a clear negative temperature coefficient (NTC) behavior for both compositions, characteristic of thermally activated hopping conduction. The Cr-substituted ferrite exhibited modified activation energy and enhanced thermal stability, with a resistivity drift of approximately 3.3% after 1000 h at 560 K. The obtained results indicate that Cr substitution significantly influences the electrical transport behavior of CuFe₂O₄ ferrites and suggest that CuFe_1.5Cr_0.5O₄ may be considered as a promising candidate for NTC thermistor applications operating at elevated temperatures.