On the phases of Polonium

The thermodynamical properties of the main phases of metallic polonium are examined using Density Functional Theory. The exceptional nature of the solid-solid phase transition of $\alpha$ to $\beta$ Po is underlined: it induces a lowering in symmetry, from cubic to rhombohedral, with increasing temperature. This is explained as the result of a delicate balance between relativistic and entropic effects. Overall agreement with existing experimental data is very good by state-of-the-art standards. The phonons of Po present Kohn anomalies, and it is shown that the effect of spin-orbit interactions is the inverse of that in normal metals: due to the non-spherical nature of the Fermi Surface, spin-orbit effects reduce nesting and harden most phonon frequencies.