Structure and magnetism in LaCoO3.

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

Local vibrations and negative thermal expansion in ZrW2O8.

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.

Lead is not off center in PbTe: the importance of r-space phase information in extended x-ray absorption fine structure spectroscopy.

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.