Influence of Sn~(4+) and Sn~(4+)/Mg~(2+) doping on structural features and visible absorption properties of α-Fe _2O_3 hematite

摘要

Pure, Sn-doped and Mg/Sn co-doped α-Fe_2O_3 hematite samples were synthesized by precipitation process. Fe_2O _3 is the most popular red mineral pigment which is used largely in traditional ceramics, tar and concrete. The compounds were characterized by powder X-ray diffraction (XRD), scanning transmission electronic microscopy (energy dispersive X-ray cartography), Mssbauer spectroscopy, magnetic investigations versus temperature and visibleNIR spectroscopy. Both ~(57)Fe and ~(119)Sn Mssbauer analyses combined with rietveld XRD refinements are the ideal techniques to characterize tiniron oxides. Hence, thanks to these techniques it was shown how the synthesis temperature influences directly the grain size and the dopants concentration limit which can be incorporated into the host hematite matrix. The stabilization of these tetravalent and divalent dopants into the hematite framework leads to reduce the crystal growth and to limit the (AF) ordering due to the formation of cationic vacancies. The study of the Morin magnetic transition emphasizes this demonstration. In a second part, the influence of the dopants incorporation on the material color was investigated in order to show which key parameters allow improving the red color saturation of iron oxides. In order to improve the red color of the hematites, it was shown that the introduction of cationic vacancieslimiting the octahedral distortion thanks to the interruption of the dissymmetric metalmetal orbital couplingis the key point. Vacancies are created by Sn~4, doping for an increase of the introduced Sn~4 concentration; it acts to the detriment of the color saturation.