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We present an x-ray pair distribution function (XPDF) analysis and extended x-ray absorption fine structure (EXAFS) data for ZrW2O8 (10-500 K) with a focus on the stiffness of the Zr-O-W linkage. The XPDF is highly sensitive to W-Zr and W-W correlations, but much less so to O-O or W-O correlations. The Zr-W peak in the XPDF data has a weak temperature dependence and, hence, this linkage is relatively stiff and does not permit bending of the Zr-O-W link. We propose that the low energy vibrational modes that lead to negative thermal expansion involve correlated rotations of ZrO6 octahedra that produce large 〈111〉 translations of the WO4 tetrahedra, rather than a transverse motion of O atoms that imply a flexible Zr-O-W linkage. [ABSTRACT FROM AUTHOR]

PbTe is a well-known thermoelectric material. Recent x-ray total scattering studies suggest that Pb moves off center along 100 in PbTe, by ∼0.2 Å at 300 K, producing a split Pb-Te pair distribution. We present an extended x-ray absorption fine structure spectroscopy (EXAFS) study of PbTe (and Ti doped PbTe) to determine if Pb or Te is off center. EXAFS provides sensitive r- or k-space phase information which can differentiate between a split peak for the Pb-Te distribution (indicative of off-center Pb) and a thermally broadened peak. We find no evidence for a split peak for Pb-Te or Te-Pb. At 300 K, the vibration amplitude for Pb-Te (or Te-Pb) is large; this thermally induced disorder is indicative of weak bonds, and the large disorder is consistent with the low thermal conductivity at 300 K. We also find evidence of an anharmonic potential for the nearest Pb-Te bonds, consistent with the overall anharmonicity found for the phonon modes. This effect is modeled by a "skew" factor (C3) which significantly improves the fit of the Pb-Te and Te-Pb peaks for the high temperature EXAFS data; C3 becomes significant above approximately 150-200 K. The consequences of these results will be discussed. [ABSTRACT FROM AUTHOR]

When PbTe, a good thermoelectric material, is doped with a few percent Tl, the figure of merit ZT=TS2/(ρκ) [S is the Seebeck coefficient, ρ the electrical resistivity, and κ the thermal conductivity] is dramatically improved. The maximum value of ZT occurs for approximately 2% Tl, but the factors limiting ZT are as yet poorly understood. From a detailed local structure study of PbTe:Tl using the extended x-ray absorption fine structure (EXAFS) technique, we find that Tl substitutes primarily as Tl(+1) on the Pb site, with no evidence for any solubility issue, any significant fraction of Tl(+3), or any Tl interstitials. However it is not a simple substitution as there is evidence for increasing Te vacancies on neighboring sites with increasing Tl concentration, x. In addition there is also increasing disorder with x—Tl-Te bond length disorder as well as the vacancy defects—that will scatter the hole carriers and begin to increase the electrical resistivity in spite of an increase in hole concentration. This increased disorder is likely an important factor limiting ZT. [ABSTRACT FROM AUTHOR]

We present a temperature-dependent, extended x-ray absorption fine structure (EXAFS) analysis of the filled skutterudite compounds NdOs4Sb12, PrOs4Sb12, and EuOs4Sb12. Although the interpretation of recent ultrasonic measurements suggested off-center displacements for Nd and Pr in NdOs4Sb12 and PrOs4Sb12, the EXAFS analysis shows that the Nd-Sb, Pr-Sb, and Sb-Sb peaks are well ordered. Surprisingly, however, the second-neighbor Nd-Os and Pr-Os peaks are highly disordered, even at low temperatures, and the Os-Os peak also has some disorder in the Nd and Pr systems. In contrast to the anomalous results for the Pr and Nd samples, neither the Eu-Sb, Eu-Os, nor the Os-Os peak is disordered at low temperature for EuOs4Sb12. For all three systems within estimated errors, the rare-earth atom is on-center inside the Sb cage. We propose that for the Nd and Pr compounds, the Os cage distorts, with some of the Os atoms moving either towards or away from the Nd or Pr atoms, such that the Nd-Sb, Pr-Sb, and Sb-Sb pair distances have very little disorder. Some possible distortion models are suggested and the possible ramifications for thermoelectric and transport properties are discussed. [ABSTRACT FROM AUTHOR]

The local structure of two skutterudite families--CeM4As12 (M = Fe, Ru, Os) and LnCu3Ru4O12 (Ln = La, Pr, and Nd)--have been studied using the extended x-ray absorption fine structure (EXAFS) technique with a focus on the lattice vibrations about the rare-earth "rattler atoms" and the extent to which these vibrations can be considered local modes, with the rattler vibrating inside a nearly rigid cage. X-ray absorption data at all the metal edges were collected over a temperature range of 4 to 300 K and analyzed using standard procedures. The pair distances from EXAFS results agree quite well with the average structure obtained from diffraction. The cage structure is formed by the M and As atoms in CeM4As12 and by Cu, O, and Ru atoms in LnCu3Ru4O12. Although some of the bonds within the cage are quite stiff (correlated Debye temperatures, θcD, are ~500 K for CeM4As12 and above 800 K for LnCu3Ru4O12), we show that the structure is not completely rigid. For the rattler atom the nearest-neighbor pairs have a relatively low Einstein temperature, θE: ~100-120 K for Ce-As and ~130 K for Ln-O. Surprisingly, the behaviors of the second-neighbor pairs are quite different: for CeM4As12 the second-neighbor pairs (Ce-Af) have a weaker bond while for LnCu3Ru4O12 the Ln-Ru second-neighbor pair has a stiffer effective spring constant than the first-neighbor pair. In addition, we show that the As4 or CuO4 rings are relatively rigid units and that their vibrations are anisotropic within these cubic structures, with stiff restoring forces perpendicular to the rings and much weaker restoring forces in directions parallel to the rings. Consequently vibrations of the rings may also act as "rattlers" and help suppress thermal conductivity. In general neither the rigid-cage approximation nor the simple reduced-mass approximation are sufficient for describing rattler behavior. [ABSTRACT FROM AUTHOR]