This stuff fascinates me since I did my PhD thesis (long long ago) on quasicrystals, a somewhat related area – the weird thing if you look across the periodic table at the metals is the range of crystal structures they have; many are very simple, but some are hideously complex (mercury, gallium, plutonium!) and when you get into alloying them even stranger things happen. Once upon a time the weird crystal structures were blamed on effects from d electrons (manganese, for example, was blamed for causing aluminum to “go bad” in the first aluminum-magnanese quasicrystals). My thesis work was an attempt to show that these sorts of things naturally happen if you take the effective metallic interactions seriously – more on my take on that in this paper.

Anyway, finding this at high pressures now in sodium (a 16-atom unit cell crystal structure, and even more complicated stuff), and apparently also in lithium (which is even superconducting at quite high temperatures) means you can’t blame the d electrons for the low-symmetry structures (which are also there for the heavier alkali elements like cesium).

The new data for sodium are unique in that they imply a liquid that is denser than the solid phase over a huge range of pressures and temperatures along the melting curve. It’ll be interesting to see what other surprises nature has in store for us!