High-pressure phase transitions in rare earth metal thulium to 195 GPa

We have performed image plate x-ray diffraction studies on a heavy rare earth metal, thulium (Tm), in a diamond anvil cell to a pressure of 195 GPa and volume compression V/V(o) = 0.38 at room temperature. The rare earth crystal structure sequence, hcp -> Sm-type -> dhcp -> fcc -> distorted fcc, is observed in Tm below 70 GPa with the exception of a pure fcc phase. The focus of our study is on the ultrahigh-pressure phase transition and Rietveld refinement of crystal structures in the pressure range between 70 and 195 GPa. The hexagonal hR-24 phase is seen to describe the distorted fcc phase between 70 and 124 GPa. Above 124 +/- 4 GPa, a structural transformation from hR24 phase to a monoclinic C2/m phase is observed with a volume change of -1.5%. The equation of state data shows rapid stiffening above the phase transition at 124 GPa and is indicative of participation of f-electrons in bonding. We compare the behavior of Tm to other heavy rare-earths and heavy actinide metals under extreme conditions of pressure.