Pairing in dense lithium

The light alkali metals have played an important role in the developing understanding of the electronic structure of simple metals. These systems are commonly viewed in terms of an underlying interacting electron gas permeated by periodic arrays of ions normally occupying only a small fraction of the crystal volume. The electron–ion interaction, or equivalently the pseudopotential, has been argued to be weak in such systems, reflecting the partial cancellation of nuclear attraction by the largely repulsive effects of Pauli exclusion by the core electrons. Current experiments can now achieve densities at which the core electrons substantially overlap, significantly reducing the fractional volume available to fixed numbers of valence electrons. Here we report the results of first-principles calculations1,2 indicating that lithium, the band structure of which is largely free-electron-like at ordinary densities, does not follow the intuitive expectations of quantum mechanics by becoming even more free-electron-like at higher densities. Instead, at high pressure its electronic structure departs radically from nearly free-electron behaviour, and its common symmetric structure (body-centred cubic, b.c.c.) becomes unstable to a pairing of the ions. Once paired, lithium possesses an even number of electrons per primitive cell which, although not sufficient, is at least necessary for insulating (or semiconducting) behaviour within one-electron band theory.