Orthorhombic (Fe2TiO5)-monoclinic (Cr2TiO5) solid solution series: Synthesis by gel routes, coloring and NIR reflectivity evaluation

CrxFe2-xTiO5 compositions of the solid solutions series from pseudobrookite Fe2TiO5 to monoclinic Cr2TiO5 have been prepared by the ceramic route and gel methods. At 1400 °C, pseudobrookite crystallizes in the x = 0–0.4 range, both pseudobrookite and monoclinic Fe-Cr2TiO5 coexist at x = 0.5, while in the range x = 0.7–1.5 monoclinic Fe-Cr2TiO5 crystallizes. Powders were 5 wt% glazed within a double-firing frit and the composition with x = 0.1 fired at 1000 °C exhibits the best red color (L*a*b* = 43.2/18.8/12.5), showing an intense band at 520 nm in the UV–Vis–NIR absorption spectrum associated with Cr4+ in octahedral coordination. The unit cell volume of the pseudobrookite decreases smoothly with the chromium amount, associated with the entrance of Cr4+ replacing Ti4+, while the volume of the Fe-Cr2TiO5 unit cell increases with the entrance of Fe3+ replacing Cr3+. The use of flux agents, the compaction pressure and the gel methods increase the crystallization of pseudobrookite but do not improve the reddish color, which is associated with the concentration of Cr4+ in the pigment. The optimized pigment composition Cr0.1Fe1.9TiO5 also shows a high NIR reflectance (55% in powder, 54% in glazed sample)