Unusual Compressive Behaviour in the Alkali Metals

The alkali metals were long considered prototypical free-electron metals, with their simple bcc structure at ambient. However, high-pressure experiments have revealed several complex phases such as host-guest structures (Na-V, K-III, Rb-IV), large unit cells (Li-V, Rb-III), and electrides (Na-VI, K-VII, Rb-VII). While several phase transitions in the alkali metals are accompanied by volume discontinuities, we have recently identified significant anomalies in the compression curves of these elements, indicating that even within phases there may be changes in the electronic structure. We explore these anomalies with a particular focus on Rb, using linearisation plots based on the APL equation of state form which offer a poweful way to identify changes in the compressive behaviour.