 Europium forms stable compounds with all of the chalcogens , but the heavier chalcogens ( S , Se , and Te ) stabilize the lower oxidation state . Three oxides are known : europium ( II ) oxide ( EuO ) , europium ( III ) oxide ( Eu2O3 ) , and the mixed @-@ valence oxide Eu3O4 , consisting of both Eu ( II ) and Eu ( III ) . Otherwise , the main chalcogenides are europium ( II ) sulfide ( EuS ) , europium ( II ) selenide ( EuSe ) and europium ( II ) telluride ( EuTe ) : all three of these are black solids . EuS is prepared by sulfiding the oxide at temperatures sufficiently high to decompose the Eu2O3 : 
