Temperature Dependent Local Structure Research Articles

Temperature dependent local structure of LiCoO2 determined by in-situ Co K-edge X-ray absorption fine structure (EXAFS)

Recently, lithium cobalt oxide (LiCoO2) has a great attention as active cathode material lithium ion batteries. In this work, LiCoO2 powder was prepared by solid state reaction and co-precipitation method. The synthesis parameters and phase formation behavior were investigated in details via several techniques, including thermal analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). A change in local structure depending on temperature around Co atoms was investigated using in-situ extended X-ray absorption fine structure (EXAFS). The temperature ranged between 300 °C and 700 °C under air condition was applied for this study.

Temperature dependent local atomic displacements in NaSn2As2 system

NaSn2As2 is mechanically exfoliable layered van der Waals (vdW) Zintl phase that is getting interesting due to its low thermal conductivity and recently observed superconductivity. Here, we have investigated the temperature dependent local structure of NaSn2As2 by a combined analysis of As K-edge and Sn K-edge extended x-ray absorption fine structure measurements. The system is intrinsically disordered with the interatomic distances largely consistent to those estimated by average structure measurements. The stretching force constants of different bond distances have been determined using temperature dependent mean square relative displacements. The Sn–As distance is the strongest bond in this system, having covalent nature, unlike the weaker interlayer distances which are characterized by vdW type bonding. Among them, As–Na distance is slightly weaker than Sn–Sn(i) below ∼200 K and tends to get stronger above this temperature. The anomalous behavior of As–Na bond suggests that the mechanical exfoliation in this system is likely to be temperature dependent. The anomaly in the interlayer atomic correlations may be due to a charge density wave-like instability around this temperature, indicated by earlier experiments. The local structure and disorder are discussed in relation to the physical properties of NaSn2As2.

Temperature dependent local structure in BaTiO3 single crystal

ABSTRACTThe temperature dependent local structure of BaTiO3 single crystal was investigated by Synchrotron X-ray Absorption Spectroscopy (XAS) technique. From the X-ray Absorption Near-Edge Structure (XANES) measurements at Ti K-edge, it was seen that the local structure around Ti atom gradually changed with increasing temperature, showing the phase transition from tetragonal to cubic. A comparison between the measured and calculated XANES spectra revealed the gradual change of local structure as a result of the decreasing c/a ratio.

EXAFS study of the local structure of crystalline and nanocrystalline Y2O3 using evolutionary algorithm method

Temperature-dependent local structure and lattice dynamics of yttria (Y2O3) were studied by X-ray absorption spectroscopy. Novel method, which combines the reverse Monte Carlo and evolutionary algorithm techniques, was applied for the analysis of extended X-ray absorption fine structure at the Y K-edge. This approach allowed us to reconstruct 3D atomic structure models of crystalline and nanocrystalline Y2O3.

Temperature dependent local structure of LiCoO2 nanoparticles determined by Co K-edge X-ray absorption fine structure

Temperature dependent Co K-edge extended X-ray absorption fine structure is used to investigate local disorder in LiCoO2 nanoparticles. We find that the nanostructuring has direct influence on the bondlength characteristics. The results reveal a substantial decrease in the force constant of Co–O bonds (the Co–O bonds becoming more flexible), while that for the Co–Co bonds showing hardly any change (or increases slightly) in LiCoO2 nanoparticles with respect to the bulk. Therefore, both random disorder and Co–O bondlength flexibility should be the factors to limit the battery characteristics of the LiCoO2 nanoparticles.

Localversusaverage structure: a study of neighborite (NaMgF3) utilizing the pair distribution function method for structure determination

The temperature-dependent local structure (< 2 nm) of neighborite (NaMgF3) is probed through least-squares refinement of structure models fit to the pair distribution function [G(r)] derived from the total high-energy X-ray scattering of sample powders. In contrast to previous temperature-dependent structure models obtained through Rietveld refinement and statistical modelling of powder diffraction data, it is found that the average Mg—F bond length, corresponding to a ∼2 Å peak in theG(r), increases between 323 and 1123 K. At each temperature, asymmetry in this peak is consistent with an orthorhombic (Pbnm) perovskite local structure, allowing three unique Mg—F values and deformation of MgF6octahedra. Defined by the three orthogonal Mg—Mg distances, the pseudo-cubic unit cell of local structure models becomes metrically tetragonal and cubic at temperatures greater than ∼623 and 1038 K, respectively. A discontinuity in the temperature dependence of the fluorine atomic displacement parameters at 1038 K suggests thermal activation of new vibrational modes in NaMgF3at high temperature, consistent with transverse vibration of the bridging fluorine atoms (Mg—F—Mg).

Local structure of multiferroicTbMn2O5: Evidence for an anomalous Tb-O distribution

The temperature dependent local structure of TbMn2O5 was determined by x-ray absorption spectroscopy. An anomalous Tb-O distribution is found. At high temperature it is broad but resolves into two distinct peaks below approximately 180 K. The distributions sharpen below the Tb magnetic ordering temperature (approximately 10 K). The distortions in the Tb-O distribution, away from the Pbam structure, are consistent with rotations of the MnOx polyhedra about the c-axis and suggest that Tb-O bond polarization may play a significant role in the observed ferroelectric properties of this system.

Temperature-dependent local structure in theNb3Gesuperconductor studied by high-resolution GeK-edge EXAFS measurements

Temperature dependent local structure of ${\mathrm{Nb}}_{3}\mathrm{Ge}$ superconductor has been studied by Ge K-edge extended x-ray-absorption fine structure measurements. Correlated Debye-Waller factor of the Ge-Nb pairs shows a clear change across the superconducting transition temperature ${T}_{c}.$ This change appears to be similar to the one observed for the $\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}\ensuremath{-}{T}_{c}$ cuprates, indicating a common cause of pairing in the short coherence length superconductors. The results suggest an intimate relation between the local atomic displacements and the short coherence superconductivity, provoking models based on local electron-lattice interaction for the $\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}\ensuremath{-}{T}_{c}$ superconductivity.

Study of Temperature Dependent Local Structure by Polarized Cu K-edge EXAFS Measurements on La2-xSrxCuO4(x=0.105, 0.13, 0.20)

We have studied temperature dependent local structure of superconducting La 2- x Sr x CuO 4 (0.105, 0.13, 0.20) single crystals by Cu K-edge extended x-ray absorption fine structure (EXAFS) measurements with polarization parallel to the in-plane Cu–O bonds. We find that, while underdoped crystals ( x =0.105, 0.13) show anomalous temperature dependence, similar to the case of optimally doped system ( x =0.15), overdoped crystal ( x =0.20) does not reveal such anomaly. Correlated Debye–Waller factor (DWF) of the Cu–O bonds (distance broadening) has been used as an order parameter to determine characteristic local displacements in the CuO 2 plane. The amplitude of temperature dependent step-like increase in the DWF at low temperature decreases with increasing doping. It has been discussed that decreasing electron–lattice interaction with increasing doping, shown by angle resolved photoemission measurements, is closely related to the evolving anomalous local CuO 2 distortion and charge inhomogeneity with doping in the CuO 2 plane of high T c oxides.