Unified High-Pressure Phase-Transition Sequence in the f-Electron Metals: oF16→oF8 Transition in Terbium.

The lanthanide elements have long been reported as having a common series of structural phase transitions on compression, but recent diffraction studies have revealed two series of transitions: one in the low-Z lanthanides (Nd and Sm) involving an 8-atom orthorhombic structure (oF8), also seen in some actinide elements, and another in the high-Z lanthanides (Gd, Tb, Dy, and Ho) involving an isosymmetric 16-atom structure (oF16), unique to the lanthanides. No element has yet been found to exhibit both structures. We have followed the structural evolution of Tb to 308 GPa and find that oF16-Tb transforms to oF8-Tb at 234 GPa, which is stable to at least 308 GPa. This transition is confirmed by theoretical results which show the oF8 phase becoming energetically favorable above 280 GPa. This observation of the oF16→oF8 transition unites the two phase-transition sequences and suggests that there is a common oF16→oF8→oC4→tI2 transition sequence in the lanthanide elements under pressure.