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.

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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 Tl 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.

The extraordinary electronic phenomena found in the Ce 1--x Yb x CoIn 5 system have attracted much interest. These phenomena include an Yb valence transition, a change in Fermi surface topology, and suppression of the heavy fermion quantum critical field at a nominal concentration x ≈ 0.2, which, surprisingly, have no discernable effect on the unconventional superconductivity and normal--state non--Fermi liquid behavior that occur over a broad range of x up to ~0.8. However, the variation of the coherence temperature T* and the superconducting critical temperature T c with nominal Yb

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.