 As reflected in the table above , samarium halides change their crystal structures when one type of halide atoms is substituted for another , which is an uncommon behavior for most elements ( e.g. actinides ) . Many halides have two major crystal phases for one composition , one being significantly more stable and another being metastable . The latter is formed upon compression or heating , followed by quenching to ambient conditions . For example , compressing the usual monoclinic samarium diiodide and releasing the pressure results in a PbCl2 @-@ type orthorhombic structure ( density 5 @.@ 90 g / cm3 ) , and similar treatment results in a new phase of samarium triiodide ( density 5 @.@ 97 g / cm3 ) . 
