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Role of Fe3+ in altering the degrees of freedom in ZnAl2O4 spinel

Distribution of Zn2+, Al3+, and Fe3+ ions on tetrahedral and octahedral sites in Fe3+-doped ZnAl2O4 nanocrystals is studied through detailed x-ray diffraction (XRD) analysis. Significant changes in the intensity ratio of (220), (311), (400), and (422) diffraction peaks upon Fe addition were perceived, whose correlation with lattice inversion, Fe3+ ions distribution, and ionic vacancies is probed through simulations and Rietveld refinement. Fe3+ ions were found to be occupying tetrahedral sites in increasing proportion with the doping concentration, leading to decreased spinel inversion. However, different orders of defects, viz., cation and anion vacancies were also found to be playing their part in the statistical distribution of cations on octahedral/tetrahedral interstices. Fe L2,3-edge x-ray absorption near edge structure spectroscopy indicated Fe3+ ions being present on tetrahedral as well octahedral interstices and distortion in cation polyhedra, while O K-edge inferred formation of different orders of ionic vacancies upon Fe addition, thereby corroborating the results from XRD. The photoluminescence study illustrated the quenched emission intensity upon Fe doping, with almost no change in emission color, and signatures of tetrahedral/octahedral Fe3+ occupancy in the lattice. Thus, this study highlights the variation in degrees of freedom associated with atomic arrangements in spinel ZnAl2O4 lattice upon Fe3+ addition.