 At ambient conditions bismuth shares the same layered structure as the metallic forms of arsenic and antimony , crystallizing in the rhombohedral lattice ( Pearson symbol hR6 , space group R3m No. 166 ) , which is often classed into trigonal or hexagonal crystal systems . When compressed at room temperature , this Bi @-@ I structure changes first to the monoclinic Bi @-@ II at 2 @.@ 55 GPa , then to the tetragonal Bi @-@ III at 2 @.@ 7 GPa , and finally to the body @-@ centered cubic Bi @-@ IV at 7 @.@ 7 GPa . The corresponding transitions can be monitored via changes in electrical conductivity ; they are rather reproducible and abrupt , and are therefore used for calibration of high @-@ pressure equipment . 
