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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.

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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.

Considerable excitement has been caused recently by the discovery that the binary boride system with stoichiometry MgB2 is superconducting at the remarkably high temperature of 39 K [1]. This potentially opens the way to even higher Tc values in a new family of superconductors with unexpectedly simple composition and structure. The simplicity in the electronic and crystal structures could allow the understanding of the physics of high-Tc superconductivity without the presence of the multitude of complicated features, associated with the cuprates. Synchrotron X-ray diffraction was used to measure the isothermal compressibility of MgB2, revealing a stiff tightly-packed incompressible solid with only moderate bonding anisotropy between intra-and inter-layer directions. These results, combined with the pressure evolution of the superconducting transition temperature, Tc establish its relation to the B and Mg bonding distances over a broad range of values.MgB 2 adopts a hexagonal crystal structure (AlB 2 -type, space group P 6/mmm) [2] which is analogous to intercalated graphite with all hexagonal prismatic sites of the primitive graphitic structure (found in hexagonal BN) completely filled and resulting in two interleaved B and Mg layers. In addition, allowing for full charge transfer from Mg to the boron 2D sheets, the latter are themselves isoelectronic with graphite.Detailed information on the properties of MgB 2 is being currently rapidly accumulated. Band structure calculations clearly reveal that, while strong B-B covalent bonding is retained, Mg is ionized and its two electrons are fully donated to the B-derived conduction band [3][4][5][6]. Superconductivity in MgB 2 is then essentially due to the metallic nature of the boron 2D sheets and the presence of strong electron-phonon interactions together with the high vibrational frequencies of the light B atoms ensure a high transition temperature [3]. Support for such a phonon-mediated BCS-type mechanism has been provided by measurements of the boron isotope effect (∆T c = 1.0 K, isotope exponent α B ∼ 0.26) [7]. In addition, T c has been found to decrease with applied pressure at the rate of -dT c /dP ∼ 1.6 K/GPa up to 1.84 GPa [8], again consistent with mediation of the pairing interaction by phonons. An alternative scenario derives from the fact that MgB 2 is hole-doped and superconductivity may be understood within a formalism developed for high-T c cuprate superconductivity [9]. Such a theory predicts a positive pressure coefficient on T c as a result of the decreasing intraplane B-B distance with increasing pressure and appears to disagree with the experimental observations. However, the response of the system may be more complex if pressure also affects the charge transfer between the B planes and Mg and will vary depending on whether the system is in the overdoped or underdoped regime.Here, we address the problem of the evolution of the structural properties of the MgB 2 superconductor with applied pressure using synchrotron X-ray powder diffraction tec...

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The low and glasslike thermal conductivity of metal-doped semiconductor clathrate compounds makes them potentially high-efficiency thermoelectric materials. The cause of this unique and remarkable property has been postulated to be due to resonant scattering of lattice phonons by localized vibrations of the dopants. We present theoretical evidence in support of this hypothesis through the analysis of electronic and vibrational interactions between dopant atoms with the host framework. In particular, the contrasting behavior of two clathrates: the glasslike thermal conductivity in Na8Si46 and the normal behavior in Cs8Sn44 can be rationalized.