First principles calculations of electronic structures and metal mobility of NaxSi46 and NaxSi34 clathrates

Energetics, geometry, electronic band structures, and charge transfer for NaxSi46 and NaxSi34 clathrates with different degrees of cavity filling by sodium, and the mobility of the Na atom inside the different cavities are studied using first principles density functional calculations within the generalized gradient approximation. The stabilization of the clathrate lattice and the cell volume variation upon the inclusion of Na (which appears to move easily in the larger cavities of NaxSi34, thus justifying the experimental observations) are discussed in connection with the onset of the repulsion between Na and Si for distances shorter than similar to3.4 Angstrom. For all degrees of filling of the different cavities examined we find that the electron population of the s orbitals in the partially ionized Na atoms increases with a decrease in the size of the cavity, and that the Na states contribute significantly to the density of states at the Fermi level and thus influence the properties of these compounds. (C) 2004 American Institute of Physics.