Structure and Optical Properties of Nickel-cobalt Ferrite Obtained by the Sol-gel Method with Participation of Auto-combustion

In this work ferrite nickel-cobalt powders were synthesised using sol-gel technology with participation of auto-combustion. After completing the auto-combustion process, only one phase which corresponds to the cubic structure of spinel space group Fd3m was obtained. It was found that the average size of coherent scattering regions does not exceed 62 nm. The dependences of the lattice parameter, X-ray density and specific surface area of the ferrite powders on the nickel content were found. It was shown that at substitution of cobalt cations by nickel cations, the latter occupy only B positions, thus displacing a part of Fe3+ into A positions. The optical properties of the powders depending on the degree of substitution of cobalt cations by nickel cations are studied. As a result of analysis of the absorption spectra it is revealed that the allowed direct transition of electrons from the valence band to the conduction band is inherent for all investigated powders. It was shown that the optical band gap increases with increasing concentration of nickel cations in the composition of ferrites.