The postulated low thermal conductivity and the possibility of altering the electronic conductivity of metal-doped clathrates with semiconducting host elements have stimulated great interest in exploring these compounds as promising thermoelectric materials. The electronic and thermal properties of the prototypical Na xSi (46) system are studied in detail here. It is shown that, despite the fact that the Na/Si clathrate is metallic, its thermal conductivity resembles that of an amorphous solid. A theoretical model is developed to rationalize the structural stability of the peculiar structural topology, and a general scheme for rational design of high efficiency thermoelectric materials is presented.
First-principles plane-wave pseudopotential and full-potential linearized-augmented plane-wave methods have been used to study the elastic and electronic properties of several potential superhard RuO 2 phases. The structures, relative stabilities, and the elastic constants and bulk moduli of these phases have been calculated within local-density approximation ͑LDA͒ and generalized gradient approximation ͑GGA͒. In RuO 2 , the LDA and GGA approximations yield smaller and larger lattice constants, respectively, for the Pa3 -RuO 2 structure. The internal structural parameter for oxygen atoms in the Pa3 structure has a volume dependence that differs from the experimental result and therefore implies a significantly different compression mechanism. The calculated bulk moduli are very similar for the fluorite and Pa3 structures and therefore apparently independent of the internal structural parameter. The structure and stability of a hypothetical orthorhombic RuO 2 phase is investigated.
The 23Na and 29Si NMR spectra of the sodium−silicon clathrate materials Na8Si46 and Na
x
Si136 (1 < x < 24)
and the parent Zintl phase NaSi have been studied in detail with a view to ironing out a number of ambiguities
in the published literature and to determining properties of these potential thermoelectric materials. Sharp
spectra are obtained only when the clathrate cages are close to fully occupied by Na, so that crystallographic
symmetry is achieved. Signals from Na in the small and large cages of both structures have been unequivocally
assigned. The pseudospherical cages give isotropic 23Na lines, whereas the other cages produce first-order
quadrupolar line shapes. Electric field gradients derived from these spectra, and ab initio calculations are in
remarkable agreement. The large Knight shifts of both types of nuclei have rather unusual temperature
dependences, which reflect the changing distributions of unpaired electron density in the conduction bands.
As the Na content of Na
x
Si136 is reduced there is drastic broadening of the 23Na and 29Si spectra due to a
random distribution of vacant cages and hence of environments, and the Si spectra shift to lower frequencies,
indicating a reduction in conduction s-electron density on the Si. Intensity data suggest a preferential loss of
Na from the large cages. XPS results show a protective coating of silica on the surface of the clathrates.
NMR, XPS, and XANES results all indicate transfer of electrons from Na to the Si framework, as predicted
by calculations. XANES shows a lowering of the absorption edge, and hence of the conduction band, of both
clathrates relative to crystalline Si.
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