Compressibility of Ce at 200 GPa

Cerium (Ce) is the most abundant of the rare-earth metals and is of great interest to the high-pressure community due to its unique behavior under pressure; it has a very complex phase diagram, including an isostructural volume collapse unique to the lanthanides and attributed to two minima in its interatomic potential. Above 13 GPa, Ce stabilizes in a body-centered tetragonal (bct, tI2) phase. Using bevelled and toroidal DACs, we have statically compressed Ce up to 360 GPa and confirm the stability of this phase up to the highest pressures. We observe an increase in compressibility between 200-250 GPa accompanied by a decline in the c/a ratio of the tI2 phase, indicating that the unit cell is becomes more cubic. The reason for these changes remains unexplored, but it is known that changes in the electronic structure, including charge localization or core-level crossing can result in axial ratio anomalies and changes in the compressibility. We discuss the experimental data showing this shift and explore the possible causes.