Influence of cobalt substitution on structural, optical, electrical and magnetic properties of nanosized lithium ferrite

A series of Co²⁺ substituted Li_0.5Co_xFe_2.5−xO₄ (x = 0.1, 0.3, 0.5) has been prepared by a citrate precursor method. The distribution of cations on A-site and B-site was studied by X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Mössbauer Spectroscopy. XRD confirmed the formation of ordered α-phase with prominent peaks at (220), (311), (400), (422), (511), (440). SEM and TEM confirmed the homogeneous formation of cubic phase with an average crystallite size of 50 nm. From FTIR studies, the bands at 603.78, 606.14 and 610.08 cm⁻¹ confirmed the formation of Fe³⁺–O²⁻ bond at tetrahedral (A-site), whereas bands at 477.25, 474.84 and 471.69 cm⁻¹ confirmed the formation of Fe³⁺–O²⁻ bond at octahedral site (B-site); shifting in frequency was observed with an increased amount of cobalt doping. Further, Raman spectra revealed the distribution of cations at tetrahedral and octahedral site by means of modes A_1g, T_2g, E_g. Mössbauer spectra with two magnetic sextets confirmed two different environments of Fe³⁺ ions. With an increase in cobalt doping, the crystallite size was observed to increase and hence an increase in relative area B/A ratio confirming the occupancy of Co²⁺ at B-site. The temperature dependence of DC resistivity was found to decrease with an increase in temperature. With an increase in cobalt substitution, DC resistivity was observed to increase from 2.32 × 10⁶ to 3.46 × 10⁷ Ω cm. A decrease in activation energy is noticed in the present investigation and this observed semiconducting behavior makes these nanomaterials suitable in NTC (negative temperature coefficient) devices. These observations were explained on various models and theories.