 Solid xenon changes from face @-@ centered cubic ( <unk> ) to <unk> close packed ( <unk> ) crystal phase under pressure and begins to turn metallic at about 140 GPa , with no noticeable volume change in the <unk> phase . It is completely metallic at 155 GPa . When <unk> , xenon looks sky blue because it absorbs red light and <unk> other visible frequencies . Such behavior is unusual for a metal and is explained by the relatively small widths of the electron bands in metallic xenon . 
