UV-Vis-NIR and microRaman spectroscopies for investigating the composition of ternary CdS1-xSex solid solutions employed as artists' pigments

Cd-based pigments share a common hexagonal wurtzite CdS structure in which cadmium and sulfur can be partially substituted to generate ternary phases with a wide range of colors from pale yellow to deep red. Among them, in the CdS1 - xSex solid solutions the substitution of sulfur by selenium decreases the valence-to-conduction energy tuning the color toward orange and red tonalities. On the basis of the wide literature concerning the study of these systems when nano-structured and our previous work regarding the yellow Cd-based pigments (based on ternary Cd1 - xZnxS), a study of the optical properties by UV-Vis-NIR absorption and emission and Raman spectroscopies has been carried out. The final goal was the determination of the solid solution stoichiometry exploiting the linear dependence of some absorption, emission and scattering properties with the pigment composition. In a first step, UV-Vis-NIR in absorption and emission and microRaman (at on- and off-resonance conditions) spectroscopies were combined for determining the electronic and vibrational behavior of commercial pigments with known composition. Then, the acquired knowledge has been successfully exploited for the identification of the solid solution stoichiometry of historical pigments. The chemical compositions determined by the spectroscopic methods were validated by PXRD measurements. In addition, the high sensitivity of Raman scattering to local compositional and structural heterogeneity allowed us to hypothesize the possible presence of quaternary solid solutions based on the substitution of cadmium by zinc (Cd1 - xZnxS1 - ySey) in the commercial pigments and the by barium (Cd1 - xBaxS1 - ySey) in the historical ones.