Study of the peierls distortion in Na8@Si-34 under pressure

Recent EXAFS experiments have shown that a Peierls distortion can occur in alkali sublattices [Brunet et al, Phys. Rev. B 61, 16550 (2000)]. This shows that even though all the alkali metals crystallise under ambient conditions in compact structures, the sodium sublattice in Na-delta@Si-34 undergoes a symmetry-breaking transition and forms dimers. In this article, the evolution of this distortion under pressure is predicted within the local density approximation of the density functional theory. We find a transition from a distorted structure to a regular diamond sublattice at 3.9 GPa, which is also discussed using a semi-empirical model.