Tuning color through sulfur and fluorine substitutions in the defect tin(II, IV) niobate pyrochlores

摘要

Sulfur and fluorine substituted Sn2Nb2O7pyrochlore 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 Sn2+cation is off-center by 0.39–0.48?? from the ideal site, and the oxygen O′ is strongly displaced to the 32eposition 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 Sn2+. Chemical analysis confirmed the existence of sulfur and fluorine, and119Sn M?ssbauer spectroscopy quantified the presence of both Sn2+and Sn4+in all compositions. The general formula representative of sulfur-containing pigments is nonstoichiometricSn2?z2+(Nb2-ySny4+)O7-z-y/2-xSxwith the color varying according tox. The fluorine compound is refined to beSn22+(Nb0·958Ti0.984Sn0.0584+)O6·376F0.206with 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. 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.