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X-ray absorption measurements at the U LIII, Ru K, and Fe K edges are reported for the hidden order (HO) material URu2-xFexSi2 (x=0, 0.05, 0.08, 0.10, 0.12, 0.15, and 0.20) as a function of x and temperature T. When Fe is substituted for Ru, the local structure about Fe shrinks slightly and the first neighbor Fe-Si bond length decreases by ≈0.05Å. More importantly excess disorder is observed below 80-100 K (the coherence temperature T∗) in plots of the Debye-Waller factor σ2 (σ is the width of the pair distribution function); at low T the data deviate from the usual Einstein or correlated-Debye model plots. This excess disorder is most prominent for the Ru-Si bond, and σ2 actually increases below 80 K. These results suggest a local orthorhombic distortion with B1g-like symmetry that develops below 80-100 K. A model that describes these local distortions is presented, and discussed in terms of other measurements that indicate a breaking of fourfold symmetry at low T. In addition, the square root of the difference between σ2(T) for the Ru-Si pair and a Debye fit to these data serves as an order parameter for this orthorhombic distortion, in the temperature range below 100 K. This quantity is a length related to a-b, the difference between the a and b lattice constants in the orthorhombic phase, and provides a connection between this distortion and T∗. X-ray absorption near edge structure (XANES) measurements also show that there are no changes in the edge positions down to 0.1 eV for any edge as a function of x, for T in the HO regime.

X-ray absorption measurements at the U LIII, Ru K, and Fe K edges are reported for the hidden order (HO) material URu2-xFexSi2 (x=0, 0.05, 0.08, 0.10, 0.12, 0.15, and 0.20) as a function of x and temperature T. When Fe is substituted for Ru, the local structure about Fe shrinks slightly and the first neighbor Fe-Si bond length decreases by ≈0.05Å. More importantly excess disorder is observed below 80-100 K (the coherence temperature T∗) in plots of the Debye-Waller factor σ2 (σ is the width of the pair distribution function); at low T the data deviate from the usual Einstein or correlated-Debye model plots. This excess disorder is most prominent for the Ru-Si bond, and σ2 actually increases below 80 K. These results suggest a local orthorhombic distortion with B1g-like symmetry that develops below 80-100 K. A model that describes these local distortions is presented, and discussed in terms of other measurements that indicate a breaking of fourfold symmetry at low T. In addition, the square root of the difference between σ2(T) for the Ru-Si pair and a Debye fit to these data serves as an order parameter for this orthorhombic distortion, in the temperature range below 100 K. This quantity is a length related to a-b, the difference between the a and b lattice constants in the orthorhombic phase, and provides a connection between this distortion and T∗. X-ray absorption near edge structure (XANES) measurements also show that there are no changes in the edge positions down to 0.1 eV for any edge as a function of x, for T in the HO regime.

Extraordinary electronic phenomena including an Yb valence transition, a change in Fermi surface topology, and suppression of the heavy fermion quantum critical field at a nominal concentration0.2 have been found in thesystem. These phenomena have no discernable effect on the unconventional superconductivity and normal-state non-Fermi liquid behaviour that occur over a broad range ofup to0.8. However, the variation of the coherence temperatureand the superconducting critical temperaturewith nominal Yb concentrationfor bulk single crystals is much weaker than that of thin films. To determine whether differences in the actual Yb concentration of bulk single crystals and thin film samples might be responsible for these discrepancies, we employed Vegard’s law and the spectroscopically determined values of the valences of Ce and Yb as a function ofto determine the actual compositionof bulk single crystals. This analysis is supported by energy-dispersive X-ray spectroscopy, wavelength-dispersive X-ray spectroscopy, and transmission X-ray absorption edge spectroscopy measurements. The actual compositionis found to be about one-third of the nominal concentrationup to0.5, and resolves the discrepancy between the variation of the physical properties ofsingle crystals and thin films with Yb concentration. [ABSTRACT FROM AUTHOR]

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]

We present an extended X-ray absorption fine structure (EXAFS) study of the NdOs4Sb12 system at the Nd and Os L III edges. Nd is expected to be a 'rattler' atom in this skutterudite crystal and we obtain a low Einstein temperature (61 K) as expected. However, although ultrasonic measurements have suggested a large off-center displacement of Nd, the EXAFS data are inconsistent with such a model. An analysis of the first two Os-Sb peaks in the Os L III data also shows no evidence for unusual disorder. In contrast the Nd-Os peak in the Nd L III data and the Os-Os peak in the Os L III data both show significant disorder at 4 K, with the largest disorder found for the Nd-Os pair. This suggests that a distortion of the structure that involves mainly the Os atoms is present; it is likely that this distortion - which may also be dynamic - also causes the anomalies in the ultrasonic data (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

When the well know thermoelectric PbTe is doped with a few percent Tl, the figure of merit, ZT = S2 σ / κ, is dramatically improved (here S is the Seebeck coefficient, σ the electrical conductivity and κ the thermal conductivity). Tl is a valence skipper with valences of +1 and +3 and replaces Pb(+2) in PbTe. Further Tl(+1) has a lone electron pair; consequently, the atom is asymmetric and large local distortions are expected if Tl(+1) is present. Here we we present Extended X-ray Absorption Fine Structure (EXAFS) data for 1% Tl doped PbTe at the Tl and Pb L3 edges and at the Te K edge, to probe the local structure about the dopant, and host atoms. At 10 K the local structure about Pb and Te is well ordered, although disorder of the Pb-Te (Te-Pb) pair distribution function (PDF) increases rapidly with increasing T. In contrast the environment around Tl is more disordered even at 10 K within the well ordered host material. This indicates a large variation of the Tl-Te bond lengths presumably as a result of the presence of some Tl(+1). This increased local disorder will result in additional electron and phonon scattering. We discuss possible models for the disorder about Tl and for PbTe at higher temperature for which neutron PDF measurements suggest Pb moves off-center (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We present an extended X-ray absorption fine structure (EXAFS) study of the NdOs4Sb12 system at the Nd and Os L III edges. Nd is expected to be a 'rattler' atom in this skutterudite crystal and we obtain a low Einstein temperature (61 K) as expected. However, although ultrasonic measurements have suggested a large off-center displacement of Nd, the EXAFS data are inconsistent with such a model. An analysis of the first two Os-Sb peaks in the Os L III data also shows no evidence for unusual disorder. In contrast the Nd-Os peak in the Nd L III data and the Os-Os peak in the Os L III data both show significant disorder at 4 K, with the largest disorder found for the Nd-Os pair. This suggests that a distortion of the structure that involves mainly the Os atoms is present; it is likely that this distortion - which may also be dynamic - also causes the anomalies in the ultrasonic data (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [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]

Pressure broadening and shift coefficients for the ν1+ν3 band of 12C2H2 have been measured for He, Ne, and Ar at a temperature of 195 K using high resolution diode laser spectroscopy. The pressure broadening and shifts follow patterns with rotational assignment that are similar to those at room temperature but are generally larger in magnitude. The change in magnitude is qualitatively described by assuming, for each transition, a constant cross section for pressure broadening or shifting. Better agreement may be obtained for pressure broadening coefficients by using empirically determined temperature exponents; better agreement still is obtained from close coupling calculations of the pressure broadening cross sections. [ABSTRACT FROM AUTHOR]

The temperature dependence of the hexagonal lattice parameter c of single crystal LaCoO3(LCO) with H = 0 and 800 Oe, as well as LCO bulk powders with H = 0, was measured using high-resolution x-ray scattering near the transition temperature To ≈35 K. The change of c(T) is well characterized by a power law in T − To for T>To and by a temperature independent constant for T

Lattice distortions corresponding to Ba displacements with respect to the FeAs sublattice are revealed to break the room-temperature tetragonal symmetry in Ba(Fe1-x Cox)2 As2. The displacements yield twin domains of the size of ≈10 nm. The domain size correlates with the magnitude of the local Fe magnetic moment and its non-monotonic dependence on Co concentration., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)