Most pressurized elements aren’t simple cubic

I have a comment regarding the three contributions under the heading “Some Elements Go Cubic Under Pressure” in the Letters section (Physics Today, October 2007, page 17 ).

The s orbitals of all atoms are relativistically stabilized because, unlike for all other orbitals, their probability density at the nucleus is greater than zero. The relativistic contribution to the energy increases with nuclear charge, gradually, even if nonlinearly, across the periodic table. There are many illustrations of that phenomenon in chemistry, such as the increasing redox stability of carbon-group cations from Ge ²⁺ to Sn²⁺ to Pb ²⁺, the decreasing melting point from zinc to cadmium to mercury, and the increasing difficulty of hybridization of the s and p valence orbitals in elemental structures.

The last of those three phenomena is nicely manifested in the gradual decrease in bond angle of oxygen-group elements, from sulfur to selenium to tellurium to polonium. Elemental sulfur forms nearly tetrahedral bond angles, characteristic of sp ³ hybridization, and Se and Te form similar structures with sharper angles. In polonium, the s and p orbitals will not mix anymore because the s-orbital energy is too low, so the bond angle is 90°. It is only natural that a high enough pressure would increase the relativistic stabilization of the s orbitals in Se and Te (or other s, p elements) enough to bring them into the polonium-type structure with bonds formed from unhybridized p orbitals. The relativistic effect is a factor shaping the periodic system, and it affects more than just the heaviest elements.