Rare earth sulfides pigments

Chopin T., Dupuis D., Rare Earth Metal Sulfide Pigment Compositions. U. S. Patent 5.401.309, 28 March 1995. 

A range of coloured pigments based on the sulfide of the rare earth element cerium have been recently introduced (industrial production since 1997), with colours ranging fi om yellow to red. Ceriinn sulfide itself has an intense red colour however, by the introduction of an alkaline element (lithium, sodium) in small proportions, the dark red colour is transformed into a light red to orange. 

The colour of these cerium-based compoimds can be further modified by introducing fluorine. The resulting crystal networks are two dimensional (rare earth fluorosulfides) and more stable in acid environments and more resistant to temperamre than their sulfide counterparts. By partially replacing cerium with another smaller rare earth such as samarium, intense red colours are obtained. Samarium fluorosulfides, much like their sulfide counterparts, also produce intense yellow colours and may prove to be viable pigments in the future. 

---