Frequency and temperature dependent dielectric properties of CoFe2−xYxO4: polarization and conduction mechanisms related to crystallographic symmetry and electronic transitions

Abstract

The frequency-dependent and temperature-dependent dielectric dispersion of yttrium (Y³⁺)-substituted CoFe₂O₄ (CoFe_2−xY_xO₄) has been explained with the help of structural symmetry and electronic transitions between ions in octahedral (O_h) and tetrahedral (T_d) sites. The CoFe_2−xY_xO₄ system exhibited a dielectric permittivity of 10² at room temperature. The frequency-dependent dielectric dispersion curve displayed three distinct slopes in the 100 Hz to 1 MHz range, which originated from interfacial polarizations by space charge carriers, dipolar polarization by the n-type (Fe³⁺ + e⁻ → Fe²⁺) and p-type (Co²⁺ + h → Co³⁺) charge carriers, and atomic polarization due to structural distortion. The resulting frequency dispersion curve obeyed the modified Havriliak–Negami relaxation mechanism associated with space charge conduction. The modified Havriliak–Negami relaxation mechanism suggested that the polarization in CoFe_2−xY_xO₄ originated from the collective contributions of multiple ions (Co²⁺, Fe³⁺, and Y³⁺), with the spreading of relaxation times being asymmetric. The Y³⁺ incorporation into Fe sites altered the cation distribution, octahedral symmetry, and electronic transitions (charge transfer (CT) and crystal field (CF) transitions) in the system, thereby modifying the polarization. A quantitative analysis of XRD patterns, coupled with Rietveld refinement, validated the reduced symmetry in the octahedral sites. Qualitative optical studies in the UV-vis-NIR region revealed an inter-valent charge transfer (IVCT) transition between [Co²⁺]t_2g → [Fe³⁺/Y³⁺]t_2g, an inter-sublattice charge transfer (ISCT) transition between Fe³⁺ ions in O_h and T_d sites, and CF transitions in Co²⁺ ions. AC conductivity studies indicated that the conduction process in ferrites mainly evolved through polaron tunneling and correlated barrier hopping of space charge, n-type, and p-type charge carriers in the CoFe_2−xY_xO₄ system.