 Samarium forms trivalent sulfide , selenide and telluride . Divalent chalcogenides SmS , SmSe and SmTe with cubic rock @-@ salt crystal structure are also known . They are remarkable by converting from semiconducting to metallic state at room temperature upon application of pressure . Whereas the transition is continuous and occurs at about 20 – 30 kbar in SmSe and SmTe , it is abrupt in SmS and requires only 6 @.@ 5 kbar . This effect results in spectacular color change in SmS from black to golden yellow when its crystals of films are scratched or polished . The transition does not change lattice symmetry , but there is a sharp decrease ( ~ 15 % ) in the crystal volume . It shows hysteresis , that is when the pressure is released , SmS returns to the semiconducting state at much lower pressure of about 0 @.@ 4 kbar . 
