POWDER XRD STRUCTURE REFINEMENTS AND FE-57 MOSSBAUER-EFFECT STUDY OF SYNTHETIC ZN1-XFEXAL2O4 (0-LESS-THAN-X-LESS-THAN-OR-EQUAL-TO-1) SPINELS ANNEALED AT DIFFERENT TEMPERATURES

Members of the solid solution series Zn1-xFexAl2O4 (x = 0.2, 0.4, 0.6 and 1.0) with spinel structure were synthesized by direct solid-state reaction of the simple metal oxides and metallic iron in evacuated sil ica ampoules at 1175 degrees C. Two aliquots of the single-phase spine ls obtained for each composition were annealed under vacuum at 1075 de grees C and 725 degrees C for 48 hours and then quenched in liquid nit rogen. The cation distributions of all the quenched samples were deter mined by X-ray powder diffraction, using the Rietveld method of struct ural refinement. The degree of inversion increases with iron content a nd for spinels with the same chemical composition with quenching tempe rature. The relative areas estimated for the contributions to the Moss bauer spectra of tetrahedrally- and octahedrally-coordinated Fe2+ sugg est that most of Zn2+ cations remain at the tetrahedral site, as expec ted from the relative cation site preferences. Failure to quench the e quilibrium cation distributions, suggested by deviations between the o bserved composition dependence of the cation distribution and that exp ected from the thermodynamic model of O'Neill and Navrotsky (1983, 198 4), may be explained by an enhancement of cation diffusion rates in th e Zn1-xFexAl2O4 (0 < x less than or equal to 1) spinels caused by the presence of cation vacancies. Fe3+/vacancy defects are easily formed i n these spinels due to partial oxidation of Fe2+ at high temperature.