  Polonium is the only element to crystallise into a simple cubic structure at
ambient conditions. Moreover, at high temperatures it undergoes a structural
phase transition into a less symmetric trigonal configuration. It has long been
suspected that the strong spin-orbit coupling in Polonium is involved in both
peculiarities, but the precise mechanism by which it operates remains
controversial. Here, we introduce a single microscopic model capable of
capturing the atomic structure of all chalcogen crystals: Selenium, Tellurium,
and Polonium. We show that the strong spin-orbit coupling in Polonium
suppresses the trigonal charge and orbital ordered state known to be the ground
state configuration of Selenium and Tellurium, and allows the simple cubic
state to prevail instead. We also confirm a recent suggestion based on ab
initio calculations that a small increase in the lattice constant may
effectively decrease the role of spin-orbit coupling, leading to a re-emergence
of the trigonal orbital ordered state at high temperatures. We conclude that
Polonium is a unique element, in which spins, orbitals, electronic charges, and
lattice deformations all cooperate and collectively cause the emergence of the
only elemental crystal structure with the simplest possible, cubic, lattice.
