Transparent dense sodium

Putting solids under pressure reduces the distances between their atoms, and at extremely high pressures, as electron density increases, all materials approach an ideal metal. Under pressure, then, 'simple' metals such as lithium and sodium might be expected to become increasingly better conductors. But about 10 years ago, calculations suggested that neither element responds in such a straightforward manner. Instead, it was predicted that the alkali atoms would form pairs under pressure and yield more complex structures with insulating properties. Two groups in this issue present experimental confirmation that this is the case; lithium and sodium become not more metal-like but less metal-like as pressure is applied. Ma et al. find that under about fivefold compression (200 GPa pressure), sodium transforms into a dense insulating material that is optically transparent and lacks a metallic sheen. Takahiro Matsuoka and Katsuya Shimizu show that lithium transforms from a metal to a semiconductor at twofold compression (80 GPa). This paper shows that under about 5-fold compression, sodium transforms into an optically transparent phase. Thus, about ten years after the basic effect was first predicted it is now shown that high pressure can turn an archetypal simple metal such as sodium into a dense insulating material with a rather complex structure and lacking a metallic sheen. Under pressure, metals exhibit increasingly shorter interatomic distances. Intuitively, this response is expected to be accompanied by an increase in the widths of the valence and conduction bands and hence a more pronounced free-electron-like behaviour. But at the densities that can now be achieved experimentally, compression can be so substantial that core electrons overlap. This effect dramatically alters electronic properties from those typically associated with simple free-electron metals such as lithium (Li; refs 1–3) and sodium (Na; refs 4, 5), leading in turn to structurally complex phases6,7,8 and superconductivity with a high critical temperature9,10,11. But the most intriguing prediction—that the seemingly simple metals Li (ref. 1) and Na (ref. 4) will transform under pressure into insulating states, owing to pairing of alkali atoms—has yet to be experimentally confirmed. Here we report experimental observations of a pressure-induced transformation of Na into an optically transparent phase at ∼200 GPa (corresponding to ∼5.0-fold compression). Experimental and computational data identify the new phase as a wide bandgap dielectric with a six-coordinated, highly distorted double-hexagonal close-packed structure. We attribute the emergence of this dense insulating state not to atom pairing, but to p–d hybridizations of valence electrons and their repulsion by core electrons into the lattice interstices. We expect that such insulating states may also form in other elements and compounds when compression is sufficiently strong that atomic cores start to overlap strongly.