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