Temperature-dependent X-ray Absorption Research Articles

A new cell for temperature-dependent X-ray absorption spectroscopy of liquid solutions: application to PbBr2solutions in diethylene glycol

An in situ cell has been constructed for temperature-dependent X-ray absorption experiments (EXAFS and XANES) of lead bromine (PbBr2) solutions in diethylene glycol in the temperature range from room temperature up to about 433 K. The solution is kept in a thermostated container made of carbon-reinforced teflon between two thin chemically inert quartz glass windows with a high transmission for hard X-rays. The construction of the cell ensures that these X-ray windows are thermalized so that any possible precipitation of solid products from the solution is inhibited. The cell consists mainly of two hermetically sealed teflon containers for the thermostating fluid (silicon oil) that were fitted together in such a way that a small and variable volume (approximately 2-4 cm3) for the liquid under investigation was achieved. A small thermocouple in a glass enclosure was placed in the solution to maintain temperature control and feedback to the thermostat. The cell design and its performance for temperature-dependent in situ investigations with X-rays are reported. Some preliminary results obtained for PbBr2 solutions in diethylene glycol are given.

Quantum-well structured high-Tc superconductors prepared by soft-chemical method

We have synthesized quantum-well structured high-Tc superconductors through soft-chemical processes, where Bi-based cuprate lattices are regularly interstratified with atom, molecule, and ionic salt. In these materials, superconducting (S) and nonsuperconducting (I) layers are alternately stacked to form superlattices with atomically clear-cut interfaces between them. The intercalation technique, which means a reaction of incorporating chemical species (guest) into the pre-synthesized layered compound (host), could provides an easy route to superconducting superlattices with molecular-level layer-by-layer structural feature. The joint analysis of temperature-dependent magnetic susceptibilities and X-ray absorption spectroscopy indicated that superconducting transition temperature is predominantly governed by the electronic structure of superconductive CuO2 plane.

Vibrational properties of Ga-stabilizedδ−Puby extended x-ray absorption fine structure

Temperature-dependent extended x-ray absorption fine structure (EXAFS) spectra were measured for a 3.3 at.% Ga stabilized Pu alloy over the range $T=20--300 \mathrm{K}.$ EXAFS data were acquired at both the Ga $K\mathrm{edge}$ and the Pu ${L}_{\mathrm{III}}$ edge. Curve fits were performed to the first shell interactions to obtain pair-distance distribution widths \ensuremath{\sigma} as a function of temperature. The temperature dependence of $\ensuremath{\sigma}(T)$ was accurately modeled using a correlated-Debye model for the lattice vibrational properties, suggesting Debye-like behavior in this material. Using this formalism, we obtain pair-specific correlated-Debye temperatures ${\ensuremath{\Theta}}_{\mathrm{cD}},$ of 110.7 $\ifmmode\pm\else\textpm\fi{}1.7 \mathrm{K}$ and $202.6\ifmmode\pm\else\textpm\fi{}3.7 \mathrm{K},$ for the Pu-Pu and Ga-Pu pairs, respectively. The result for the Pu-${\ensuremath{\Theta}}_{\mathrm{cD}}$ value compares well with previous vibrational studies on $\ensuremath{\delta}\ensuremath{-}\mathrm{Pu}.$ In addition, our results represent the first unambiguous determination of Ga-specific vibrational properties in PuGa alloys, i.e, ${\ensuremath{\Theta}}_{\mathrm{cD}}$ for the Ga-Pu pair. Because the Debye temperature can be related to a measure of the lattice stiffness, these results indicate the Ga-Pu bonds are significantly stronger than the Pu-Pu bonds. This effect has important implications for lattice stabilization mechanisms in these alloys.

Lattice softening in superconducting compositions of Ba(K)BiO3.

Temperature dependent x-ray absorption spectra investigation were measured for Ba(1-x)K(x)BiO3 (Bi L3-edge) with x =0.0, 0.25, 0.4, 0.5 and for BaPbO3 (Pb L3-edge). It was found that at low temperatures the Debye-Waller factor of the square diagonal Bi-Bi bond has the maximum value near the insulator-metal phase transition for the compound with x = 0.25 and x = 0.4. Temperature dependence of the Debye-Waller factor of Bi-Bi bond strongly differs from the Einstein model curve that well describes the harmonic systems (for example BaPbO3). This behavior is consistent with the strong anharmonicity of the Bi-O shell due to the double-well vibration potential reported by us earlier. Presented results point to the essential lattice softening of the superconducting compositions, which is important for the understanding of superconductivity mechanism in perovskite type oxides.

Characteristic temperature dependence of the4foccupancy in the Kondo systemCeSi2

We have used temperature dependent x-ray absorption at the Ce-L3 edge to investigate the recently discovered Kondo compound Ce2Rh3Al9. The systematic changes of the spectral lineshape with decreasing temperature are analyzed and found to be related to a change in the $4f$ occupation number, n_f, as the system undergoes a transition into a Kondo state. The temperature dependence of $n_f$ indicates a characteristic temperature of 150K, which is clearly related with the high temperature anomaly observed in the magnetic susceptibility of the same system. The further anomaly observed in the resistivity of this system at low temperature (ca. 20K) has no effect on n_f and is thus not of Kondo origin.

LOCAL STRUCTURE OF THE CHARGE ORDERED La0.5Ca0.5MnO3

Local lattice of the charge ordered La 0.5 Ca 0.5 MnO 3 has been studied by temperature dependent Mn K-edge x-ray absorption measurements, using fluorescence detection method, to explore how the local lattice distortions are associated with the charge ordering in this system. The x-ray absorption near edge structure (XANES) reveals an abrupt change in the local MnO 6 octahedral geometry across the charge ordering temperature. The extended x-ray absorption fine structure (EXAFS) analysis shows largely distorted MnO 6 octahedra with coexisting Q 3-type and Q 2-type of Jahn-Teller (JT) modes at low temperature in the commensurate charge ordered phase. We find that small JT distortions persist also in the charge disordered phase at high temperature.

The double-well oscillating potential of oxygen atoms in perovskite system Ba(K)BiO 3: EXAFS – analysis results

Temperature-dependent X-ray absorption investigations were made on the Bi L 3-edge in Ba 1− x K x BiO 3 with x=0.0, 0.4 and 0.5. For the superconducting samples ( x=0.4 and 0.5) it has been found that the local structure differs from the ideal cubic in contrast to the neutron and X-ray diffraction data. The provided analysis of the EXAFS spectra indicates that the oxygen atoms move in double-well potential produced by the existence of two non-equivalent octahedral types of the oxygen environment of bismuth. The vibrations in such a potential lead to modulations of the Bi–O lengths with low frequency which is determined by the soft oxygen octahedron rotation mode frequency. This induces strong electron–phonon interaction and may be the reason for relatively high-temperature transition ( T c∼30 K) to the superconducting state.

Probing local magnetic disorder by investigating spin dependent photoelectron scattering

We present here systematic temperature dependent magnetic extended x-ray absorption spectroscopy (MEXAFS) measurements. This fine structure can be explained by the spin dependent scattering of the photoelectron and yields information about the local magnetic surroundings of the absorbing atom. We find a strong temperature dependence at temperatures far below the Curie temperature. This shows that the intensity of the MEXAFS signal is not only determined by the value of the magnetic moment but also by the relative vibrations of the backscattering atom to the absorbing atom.

On the mechanism of superconductivity in BaBi(Pb)O 3Ba(K)BiO 3 systems: analysis of XAFS-spectroscopy data

Based on the XAFS-studies of BaPb 1− x Bi x O 3 (BPB) and Ba 1− x K x BiO 3 (BKB) systems carried out by the author from 1983 to 1996, the peculiarties of the local and the electronic structures of both compounds have been explained. It was shown that bismuth in BaBiO 3 is situated in two different positions with valence states Bi 3+ and Bi 3+ L 2 (here L 2 denotes the presence of two holes in the nearest oxygen environment), that causes subsequent difference in BiO bond strengths. Temperature dependent X-ray absorption investigations on the Bi L 3 edge point out significant local structure anomalies around Bi atoms in BaBiO 3 and BaPb 0.25Bi 0.75O 3 at low temperatures. It was found that BiO, BiBa distances abnormally increase with temperature decreasing from ∼90 K. These anomalies are accompanied by abnormal behaviour of the Debye-Waller factor σ 2 of the BiO bonds at low temperatures in contrast to the Einstein model. The nature of the above anomalies was explained in the frame of the model, taking into account the asymmetry of oxygen octahedra rotation modes which promotes understanding of the mechanism of superconductivity in BPB-BKB systems.

A temperature dependent X-ray Absorption Fine Structure study of dynamic X-site disorder in almandine: a comparison to diffraction data

X-ray Absorption Fine-Structure (XAFS) measurements in the temperature range 20-473K at the Fe K-edge of synthetic almandine (Fe3Al2Si3O12) allow us to determine the temperature dependence of the XAFS Debye-Waller (DW) factors for each of the two crystallographically independent Fe−O bond distances (Fe(1)−O and Fe(2)−O). The vibrational anisotropy of Fe2+ in the dodecahedral X-site has been documented. The XAFS DW factor of the longer Fe(2)−O bond is larger and has a greater T-dependence than the shorter Fe(1)−O bond, which remains constant over the measured T range. The XAFS DW factors are compared with the uncorrelated part of the DW factors derived from a single-crystal X-ray diffraction (XRD) study on almandine and are interpreted on the basis of the Einstein model. A comparison with diffraction data shows that the shorter Fe(1)–O bond is significantly affected by correlation in the atomic displacements.

Debye-Waller Factor Low Temperature Anomalies in BaPb1-xBixO3

Temperature dependent x-ray absorption investigations have been made on the Bi L 3 edge in BaPb 1-x Bi x O 3 with x=0.0, 0.75, 1.0. Essentially abnormal behaviour of the Debye-Waller factor σ 2 of Bi-O bonds was observed at low temperatures for BaBiO 3 and BaPb 0.25 Bi 0.75 O 3 in contrast to Einstein model. It was found that the Debye-Waller factors of both Bi-O bonds increase with temperature decreasing from 90 K along with abnormal increasing of Bi-O and Bi-Ba distances. At the same time Pb-O, Pb-Ba bonds of BaPbO 3 show normal behaviour with temperature. The nature of the anomalies is discussed in the frame of the dynamic fluctuation of the oxygen octahedra rotation mode that may give a new look to understanding of HTSC phenomenon in BaPb 1-x Bi x O 3 and Ba 1-x K x BiO 3 .

Local structure anomalies of the BaBi(Pb)O 3 system at low temperatures: an X-ray absorption study

Temperature dependent X-ray absorption investigations were made on the Bi L 3 edge in BaPb 1− x Bi x O 3 with x = 0.0, 0.75 and 1.0. Essential local structure anomalies around Bi atoms were observed in BaBiO 3 and BaPb 0.25Bi 0.75O 3 at low temperatures. It is found that BiO, BiBa distances abnormally increase with the temperature decreasing from 90 K. These anomalies are accompanied by an abnormal behaviour of the Debye-Waller factor σ 2 of the BiO bonds at low temperatures in contrary to the Einstein model. The lengths and σ 2 of the PbO and PbBa bonds of BaPbO 3 show an ordinary increase with temperature. The anomalies are shown to be due to the rotation distortion asymmetry of the oxygen octahedra owing to the difference in the strength of BiO bonds. The experimental results are discussed in the frame of the dynamic fluctuation rotation mode.

Local disorder in crystalline and amorphous germanium

Anharmonic contributions to the pair potential of near-neighbor interactions of crystalline and amorphous germanium have been determined by temperature-dependent x-ray absorption fine structure measurements. The measurements have been carried out at the K edge of Ge in the temperature range 77\char21{}450 K and analyzed by the cumulant method. In c-Ge the temperature dependence of the first four cumulants has been determined for the first three coordination shells. The radial distribution function of the nearest-neighbor atoms has been found Gaussian in the examined temperature range; the second and third shell distance distributions show a symmetric but non-Gaussian behavior even below the Debye temperature. Cumulants have been related to the force constants of the effective pair potential. The harmonic contribution to thermal disorder has been extracted from the second cumulant for each coordination shell and compared with the mean square relative displacements calculated by harmonic models of lattice dynamics; experimental results are in good agreement with theoretical calculations. In amorphous germanium the first shell radial distribution function is asymmetric even at the lowest temperatures. Static and dynamic effects to the local disorder have been separately determined and compared with the results of previous studies.

Temperature dependent EXAFS measurements on YGG during a sol-gel process

Temperature dependent X-ray absorption spectroscopy has been used as a local probe to elucidate the structure of yttrium gallium garnet produced by a sol-gel route. The treatment of our experimental data includes a cumulant expansion analysis taking into account the asymmetry of the pair distribution function resulting from disorder at high temperature.