Direct observation of a pressure-induced metal-to-semiconductor transition in lithium

Li and Na show resistance 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).