Spectroscopic active IVFe3+-VIFe3+ clusters in spinel-magnesioferrite solid solution crystals:: a potential monitor for ordering in oxide spinels

Optical absorption spectra (OAS) of synthetic single crystals of the solid solution spinel sensu stricto (s.s.)-magnesioferrite, Mg(Fe3+ Al1-y)(2)O-4 (0 < Y less than or equal to 0.3), have been measured between 12 500 and 28 500 cm(-1). Chemical composition and Fe3+ site distribution have been measured by electron microprobe and Mossbauer spectroscopy, respectively. Ferric iron is ordered to the tetrahedral site for samples with small magnesioferrite component, and this ordering is shown to increase with magnesioferrite component. The optical absorption spectra show a strong increase in band intensities with Fe3+ --> Al substitution. Prominent and relatively sharp absorption bands are observed at 25 300 and 21 300 cm(-1), while less intense bands occur at 22 350, 18 900, 17 900 and 15 100 cm(-1). On the basis of band energies, band intensities and the compositional effect on band intensity, as well as structural considerations, we assign the observed bands to electronic transitions in Fe-IV(3+) - Fe-VI(3+) clusters. A linear relationship (R-2 = 0.99) between the alpha (net) value of the absorption band at 21 300 cm(-1) and [Fe-IV(3+)].[Fe-VI(3+)] concentration product has been defined: alpha (net) 2.2 + 15.8 [Fe-IV(3+)].[Fe-VI(3+)]. Some of the samples have been heat-treated between 700 and 1000 degreesC to investigate the relation between Fe3+ ordering and absorption spectra. Increase of cation disorder with temperature is observed, which corresponds to a 4% reduction in the number of active clusters. Due to the high spatial resolution (empty set similar to 10 mum), the OAS technique may be used as a microprobe for determination of Fe3+ concentration or site partitioning. Potential applications of the technique include analysis of small crystals and of samples showing zonation with respect to total Fe3+ and/or ordering.