Study of charge transfer mechanisms in cobalt-substituted copper ferrites

Abstract In this study, Co x Cu 1-x Fe 2 O 4 (x = 0.0–1.0) spinel ferrites were synthesized by the citrate sol–gel autocombustion method and systematically investigated to clarify composition-dependent charge transport mechanisms. X-ray diffraction and FTIR confirmed spinel formation and revealed systematic cation redistribution with cobalt substitution. X-ray photoelectron spectroscopy (XPS) further verified the coexistence of Fe 3+ /Fe 2+ and Co 2+ /Co 3+ redox couples together with oxygen-vacancy–related states, providing direct evidence of multiple charge transfer pathways. Temperature-dependent conductivity showed thermally activated behavior, with a slope change above 373 K in Co-rich samples (x ≥ 0.8), signaling a transition in the dominant conduction process. At low temperatures (298–348 K), transport was directed by grain boundary and defect-related hopping, while long-range small-polaron hopping between Fe 2+ /Fe 3+ dominated at elevated temperatures. Increasing Co content introduced additional Co 2+ /Co 3+ valence fluctuations, enhancing localized dielectric relaxation and structural disorder. AC conductivity analysis demonstrated decreasing activation energy with increasing frequency, consistent with enhanced carrier mobility. The data were successfully fitted using Jonscher’s universal power law, its modified form, and the Almond–West model. Fitting revealed a composition-driven increase in onset frequency ω H from ~ 4 kHz in CuFe 2 O 4 to ~ 87 kHz in CoFe 2 O 4 , together with anomalously high dispersion parameters (s > 1, m > 1) in Co-rich samples, indicating strong interfacial polarization and dipolar relaxation. A distinct low-frequency break near 10 Hz in CuFe 2 O 4 was attributed to structural transformation. The combined structural, spectroscopic, and electrical analyses demonstrating a clear evolution from long-range Fe 2+ /Fe 3+ hopping in Cu-rich ferrites to localized relaxation and interfacial polarization in Co-rich members. This tunability, linked to cation distribution and oxygen-vacancy formation, positions Co x Cu 1-x Fe 2 O 4 as a promising spinel system for high-frequency dielectric, EMI shielding, and microwave absorption applications.