"Electronic Structure of the Na16Rb8Si136 and K16Rb8Si136 Clathrates"

We have calculated the electronic structure and the equations of state of the Na16Rb8Si136 and K16Rb8Si136 clathrates. These compounds are based on the type II silicon clathrate structure. The smaller Na or K atoms occupy the 20-atom cages, while the Rb atoms occupy the 28-atom cages. Equation of state, electronic band structure, and density of states calculations were performed using density functional theory in the local density approximation LDA. The “guest” impurity atoms inside the cages modify the material electronic structure. Guest atom electrons occupy the Si136 conduction band states, resulting in a Fermi level shift into the conduction band of the “parent” Si136 framework. In qualitative agreement with the rigid-band model, the band structures display no major modifications due to the inclusion of the alkali metal guests. However, the electronic densities of states of the filled clathrates show two sharp peaks and a dip near the Fermi level. This feature may help to qualita