Influence of Sn4+ and Sn4+/Mg2+ doping on structural features and visible absorption properties of α-Fe2O3 hematite

Pure, Sn-doped and Mg/Sn co-doped α-Fe2O3 hematite samples were synthesized by precipitation process. Fe2O3 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), Mössbauer spectroscopy, magnetic investigations versus temperature and visible–NIR spectroscopy. Both 57Fe and 119Sn Mössbauer analyses combined with rietveld XRD refinements are the ideal techniques to characterize tin–iron 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 vacancies—limiting the octahedral distortion thanks to the interruption of the dissymmetric metal–metal orbital coupling—is the key point. Vacancies are created by Sn4+, doping for an increase of the introduced Sn4+ concentration; it acts to the detriment of the color saturation.