Spectroscopic active Fe-IV(3+)-Fe-VI(3+) 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 meas ured by electron microprobe and Mossbauer spectroscopy, respectively. Ferri c iron is ordered to the tetrahedral site for samples with small magnesiofe rrite component, and this ordering is shown to increase with magnesioferrit e component. The optical absorption spectra show a strong increase in band intensities with Fe3+ --> Al substitution. Prominent and relatively sharp a bsorption bands are observed at 25 300 and 21 300 cm(-1), while less intens e 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 intens ity, as well as structural considerations, we assign the observed bands to electronic transitions in Fe-IV(3+) - Fe-VI(3+) clusters. A linear relation ship (R-2 = 0.99) between the alpha (net) value of the absorption band at 2 1 300 cm(-1) and [Fe-IV(3+)].[Fe-VI(3+)] concentration product has been def ined: alpha (net) 2.2 + 15.8 [Fe-IV(3+)].[Fe-VI(3+)]. Some of the samples h ave been heat-treated between 700 and 1000 degreesC to investigate the rela tion between Fe3+ ordering and absorption spectra. Increase of cation disor der with temperature is observed, which corresponds to a 4% reduction in th e 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 deter mination 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.