On the phases of Polonium

TL;DR

This study uses Density Functional Theory to analyze the thermodynamic properties and phase transitions of polonium, revealing unique relativistic and entropic effects influencing its solid phases and phonon behavior.

Contribution

It provides a detailed theoretical analysis of polonium's phases, highlighting the role of relativistic and entropic effects in its phase transition and phonon anomalies.

Findings

Good agreement with experimental data

Relativistic effects influence phonon frequencies

Solid-solid phase transition involves symmetry lowering

Abstract

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.