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

Abstract

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 compressionV/Vo = 0.38 at room temperature. The rare earth crystal structure sequence, distorted fcc, is observed in Tm below 70 GPa with the exception of a pure fcc phase. Thefocus of our study is on the ultrahigh-pressure phase transition and Rietveld refinement ofcrystal structures in the pressure range between 70 and 195 GPa. The hexagonalhR- 24 phase is seen to describe the distorted fcc phase between 70 and 124 GPa. Above124 ± 4 GPa, a structuraltransformation from hR 24 phase to a monoclinic C 2/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 GPaand is indicative of participation of f-electrons in bonding. We compare the behavior of Tmto other heavy rare-earths and heavy actinide metals under extreme conditions of pressure.