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

Abstract

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 Na₈@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.

Keywords:

• Clathrate
• Peierls distortion
• Expanded metals
• Tight-binding model

Acknowledgements

We gratefully acknowledge Dr. A. San Miguel (University Claude Bernard, Lyon, France) for invaluable discussions. This work was supported by FNRS Grant No. 9.4565.96F.