Linking crystal chemistry and physical properties of natural and synthetic spinels: an UV-VIS-NIR and Raman study

The present work deals with the detailed study of multiple oxides with spinel structure with the aim to explore two among their variety of useful physical properties: optical and vibrational properties. Spinels represent very important minerals largely occurring in rocks as well as materials of high relevance from a gemological and technological point of view. To understand their physical properties a complete chemical, structural and spectroscopic study was performed analyzing a large amount of natural and synthetic spinels by Electron MicroProbe, Single-Crystal X-ray Diffraction, Optical Absorption and Raman spectroscopies. Regarding the optical properties, an accurate characterization on blue synthetic spinels and on natural multicolor spinels was made to investigate the cause of the various colors shown by the spinels. The synthetic blue spinels belonging to the MgAl2O4-CoAl2O4 series were successful synthetized by flux growth method. The optical spectra of the Co-spinels show electronic transitions only due to the Co2+ in tetrahedral coordination, which causes the characteristic blue colour of cobalt spinels. The multicolor spinels owe their different colors not to the major elements but to a combination of two or more minor elements such as Cr3+, V3+, Fe2+, Fe3+ and Co2+, occupying tetrahedrally (T) and/or octahedrally (M) coordinated sites in the spinel structure as also demonstrated by the recorded optical spectra. Regarding the vibrational properties, a systematic study of three synthetic solid solutions (Mg,Co)Al2O4, (Mg,Fe)Al2O4 and (Mg(Al,Cr)2O4 was carried out by Raman spectroscopy to determine the relationships between the Raman active modes and spinel crystal chemistry. On the basis of comparative inspection of the spectra, it follows that each studied end-member exhibits a Raman fingerprint with at least one peculiar peak in terms of Raman shift and relative intensity. In addition, for each series a good relationship between a Raman active mode and spinel principal component was established. In addition to the Raman features, a study of the fluorescence features attributed to chromophoric ions observed in the Raman spectra was made to distinguish between natural, thermally-treated, and synthetic materials. Finally, a multianalytical investigation of a natural zincian spinel by electron microprobe, single-crystal X-ray diffraction, Raman and optical absorption spectroscopies was achieved to fully characterize a Zn-rich spinel approaching the endmember gahnite (ZnAl2O4) composition.