 Ytterbium has three allotropes labeled by the Greek letters alpha , beta and gamma ; their transformation temperatures are − 13 ° C and 795 ° C , although the exact transformation temperature depends on the pressure and stress . The beta allotrope exists at room temperature , and it has a face @-@ centered cubic crystal structure . The high @-@ temperature gamma allotrope has a body @-@ centered cubic crystalline structure . The alpha allotrope has a hexagonal crystalline structure and is stable at low temperatures . Normally , the beta allotrope has a metallic electrical conductivity , but it becomes a semiconductor when exposed to a pressure of about 16 @,@ 000 atmospheres ( 1 @.@ 6 GPa ) . Its electrical resistivity increases ten times upon compression to 39 @,@ 000 atmospheres ( 3 @.@ 9 GPa ) , but then drops to about 10 % of its room @-@ temperature resistivity at about 40 @,@ 000 atm ( 4 @.@ 0 GPa ) . 
