A comparative theoretical study of the Peierls distortion in type-II alkaline-doped clathrates

We show that a Peierls distortion may occur in expanded metals. When the sodium is expanded it undergoes a symmetry breaking and diatomic molecules are formed. This phenomenon is a result of an electronic instability mechanism. As shown by recent EXAFS experiments, sodium atoms included in chlathrate cages do not stay at the center of the cages. We describe this distortion by two very different methods: an ab initio study based on the local density approximation of the density-functional theory and a tight-binding description. This study is also extended to the other alkaline-doped silicon clathrates. This dimerization process is related to hydrogen dimer existence.