Temperature dependence of the cation distribution in zinc ferrite (ZnFe2O4) from powder XRD structural refinements

Zinc ferrite (ZnFe 2 O 4 ) has the cubic spinel structure with an (approximately) normal cation distribution. It was synthesised by direct reaction of the oxides at 800 ° C, and also by using a sodium tungstate flux with excess ZnO. Lattice parameters (a o ) of samples from both syntheses were emasured at room temperature, on samples annealed in air between 500 and 1200 ° C and then quenched into water, a o decreases from 8.4419(2) for samples annealed at 500 ° C, to 8.4400(2) for 950 ° C, and there is no significant difference between samples from either synthesis method. Annealing at higher temperatures causes no further decrease in a o in the quenched samples. The change of a o with time during isothermal annealing at 500 ° C established that a steady value was reached in about 24 hours. The cation distributions in quenched samples of the flux-grown material were determined by powder X-ray diffraction, using the Rietveld method of structural refinement. The degree of inversion in ZnFe 2 O 4 increases from 0.02(2) at 500 ° C to 0.19(2) at 950 ° C. Samples quenched from higher temperatures show no further increase in x, in agreement with the trend in a 0 . This is probably due to the rate of re-ordering being too fast to quench above ∼950 ° C. Fe Mossbauer spectra were collected for a few samples at 80K and 298K. All spectra show only a single doublet with hyperfine parameters characteristic of octahedral Fe 3+ . The tetrahedral Fe 3+ deduced from the XRD refinements cannot be resolved