Temperature dependence of the cation distribution in zinc ferrite (ZnFe2O4) from powder XRD structural refinements
O'neill, Hugh. St. C.
European Journal of Mineralogy Volume 4 Number 3 (1992), p. 571 - 580
published: Jun 11, 1992
manuscript accepted: Nov 25, 1991
manuscript received: Jul 26, 1991
ArtNo. ESP147050403017, Price: 29.00 €
Abstract Zinc ferrite (ZnFe2O4) 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 (a0) of samples from both syntheses were measured at room temperature, on samples annealed in air between 500 and 1200°C and then quenched into water ; a0 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 ao in the quenched samples. The change of a0 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 ZnFe2O4 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 a0. This is probably due to the rate of re-ordering being too fast to quench above ~950°C. 57Fe 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 Fe3+. The tetrahedral Fe + deduced from the XRD refinements cannot be resolved