In chemistry, a sulfoselenide is a compound containing both metal sulfides and metal selenides. Because metal sulfides and metal selenides have similar crystal structures, they exhibit some mutual solubility, forming solid solutions. Since the ionic radius sulfide of (S2−) is however much smaller than that for selenide (Se2−), the solubility ranges can be only limited. For example, pyrite (FeS2) will accept only a few percent of selenium in place of sulfur. A broader range is seen for the solid solution of cadmium sulfide and cadmium selenide. CdS is yellow and CdSe is red. The sulfoselenides of cadmium are orange. They are used as an artist's pigment.