Study of Site Occupancy in ${\rm Zn}_{x}{\rm Fe}_{3-x}{\rm O}_{4}$ Microspheres Based on Mössbauer Analysis

The crystal structure of Zn_xFe_3-xO₄ (x = 0, 0.05, 0.1, 0.2, 0.4), prepared by a solvothermal reaction technique, showed cubic spinel structure with space group FD - 3m based on Rietveld refinement. The lattice constant increased linearly with the Zn contents from to x = 0 to 0.4. Field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM) measurements showed that the size of the monodispersed particles was around 300 ~ 500 nm. With increasing Zn contents M_s and H_c values at 295 K increase with up to 0.05 and then decrease monotonously as x increases above 0.4, respectively. We have analyzed the Mössbauer spectra as 4 set with tetrahedral site and octahedral B₁, B₂, and sites at 295 K and 4.2 K. From the isomer shift values, the valence states of A, B₁, and B₃ sites were determined to be ferric (Fe³⁺), while that at B₂ site to be ferrous (Fe²⁺). The corresponding area ratio of A and B₂ sites decreased by 40 ~ 25 and 20 ~ 8%, while that of B₁, B₃ sites increased by 40 ~ 52 and 0 ~ 15%, with increasing the Zn contents. This site preference, depending on the amount of ion substituted in and sites, affects the electron hopping between Fe²⁺/Fe³⁺ ions, and changed the super-exchange interaction A - O - B and B - O - B between and sites.