Unified high-pressure phase-transition sequence in the 𝑓 -electron metals: 𝑜⁢𝐹⁡16→𝑜⁢𝐹⁡8 transition in terbium

The lanthanide elements have long been reported as having a common series of structural phase transitions on compression, but recent advances in structural analysis and synchrotron sources have revealed series of transitions: one in the low-Z lanthanides (Nd & Sm) involving an 8-atom orthorhombic structure (𝑜⁢𝐹⁡8
), also seen in some actinide elements, and another in the high-Z lanthanides (Gd, Tb, Dy & Ho) involving an isosymmetric 16-atom orthorhombic structure (𝑜⁢𝐹⁡16
), unique to the lanthanide elements. No element, neither lanthanide nor actinide, has yet been found to exhibit both structures. We have followed the structural evolution of Tb to 308 GPa and find that 𝑜⁢𝐹⁡16-Tb transforms to 𝑜⁢𝐹⁡8-Tb with an onset at 234 GPa, which is stable to at least 308 GPa. This transition is confirmed by theoretical results which show the 𝑜⁢𝐹⁡8 phase becoming energetically favorable above 280 GPa. This observation of the 𝑜⁢𝐹⁡16→𝑜⁢𝐹⁡8 transition unites the two phase transition sequences and suggests that there is a common 𝑜⁢𝐹⁡16→𝑜⁢𝐹⁡8→𝑜⁢𝐶⁢4→𝑡⁢𝐼⁢2 transition sequence in the lanthanide elements under pressure.