Tuning color through sulfur and fluorine substitutions in the defect tin(II, IV) niobate pyrochlores
In this study, sulfur and fluorine substituted Sn 2 Nb 2 O 7 pyrochlore compounds were synthesized to produce brilliant orange/red colors as an environmentally friendly alternative to other toxic inorganic pigments. Neutron structure refinements indicate that the studied compositions crystallize in the pyrochlore structure with a large number of cation and/or anion vacancies and significant local structural disorder. The cubic unit cell decreases with increasing sulfur and fluorine content. The Sn 2+ cation is off-center by 0.39–0.48 Å from the ideal site, and the oxygen O' is strongly displaced to the 32e position by an extraordinarily large distance that has never been reported for a pyrochlore type phase. The unprecedented displacive disorder and unusual lattice evolution upon sulfur substitution are presumably driven by the strong lone pair stereoactivity associated with Sn 2+ . Chemical analysis confirmed the existence of sulfur and fluorine, and 119 Sn Mössbauer spectroscopy quantified the presence of both Sn 2+ and Sn 4+ in all compositions. The general formula representative of sulfur-containing pigments is nonstoichiometric Sn$2+\atop{2-z}$ (Nb 2-y Sn$4+\atop{y}$)O 7-z-y/2-x S x with the color varying according to x. The fluorine compound is refined to be Sn$2+\atop{2}$(Nb 0·958 Ti 0.984 Sn$4+\atop{0.058}$)O 6·376 F 0.206 with exclusively anion vacancies. Optical characterization was performed through diffuse reflectance and color meter measurements. The band gap is determined to be in the range of 2.35 to 2.05 eV as sample color changes from yellow to orange/red. Finally, all powder samples show 60–95% reflectance in the near-infrared (NIR) region, adding new functionality to the application of these compounds as “cool” pigments.