Temperature Dependence Of Local Structure Research Articles

Pressure and Temperature Dependence of Local Structure and Dynamics in an Ionic Liquid.

A detailed understanding of the local dynamics in ionic liquids remains an important aspect in the design of new ionic liquids as advanced functional fluids. Here, we use small-angle X-ray scattering and quasi-elastic neutron spectroscopy to investigate the local structure and dynamics in a model ionic liquid as a function of temperature and pressure, with a particular focus on state points (P,T) where the macroscopic dynamics, i.e., conductivity, is the same. Our results suggest that the initial step of ion transport is a confined diffusion process, on the nanosecond timescale, where the motion is restricted by a cage of nearest neighbors. This process is invariant considering timescale, geometry, and the participation ratio, at state points of constant conductivity, i.e., state points of isoconductivity. The connection to the nearest-neighbor structure is underlined by the invariance of the peak in the structure factor corresponding to nearest-neighbor correlations. At shorter timescales, picoseconds, two localized relaxation processes of the cation can be observed, which are not directly linked to ion transport. However, these processes also show invariance at isoconductivity. This points to that the overall energy landscape in ionic liquids responds in the same way to density changes and is mainly governed by the nearest-neighbor interactions.

Temperature Dependence of Local Structure of Layered Cathode Material 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 during Charge and Discharge Process

INTRODUCTION In the last decades, LiCoO2 is the most important cathode material for lithium-ion batteries because of its well-balanced electrochemical performance. However, cobalt is costly, toxic and unsatisfactory on safety. Thereby, many researchers have devoted their efforts to discover a cheaper, safer and higher capacity cathode material for more than 10 years. In this regard, the solid solutions of Li2MnO3 and LiMO2(M= Mn, Ni, Co, etc.) have attracted attention over the years because they deliver high discharge capacity of over 200 mAh/g, whereas LiCoO2 delivers a discharge capacity of about 150 mAh/g. In our laboratory, we have focused on xLi2MnO3-(1-x)LiMO2(M= Mn, Ni, Co) with x=0.4 and 0.5 because of the good electrochemical performance, and then performed average and local crystal structure analysis on the materials1). However, the influence of operating temperature has not been studied. In this work, we reported on temperature dependencies of average, local and electronic structures of 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 during charge and discharge process by using neutron and synchrotron X-ray total scatterings. In addition, we newly observed the electrode by TEM and STEM/EDX, and compared the result with those of the crystal structure analysis. EXPERIMENTAL The preparation of the Li1.667Mn0.5Ni0.1667Co0.1667O2, corresponding to 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2, was synthesized by co-precipitation method. The product was identified by powder XRD. The composition of lithium and transition metal contents in the active material was determined by ICP. To study change of crystal and electronic structure of this cathode material during charge and discharge process at room and high temperatures (25°C and 60°C), we prepared some cathodes with various depth of discharge in first cycle and then measured the synchrotron X-ray diffraction (BL02B2, BL19B2, SPring-8) and neutron diffraction patterns (BL20, J-PARC) of these samples. The data was analyzed with the Rietveld technique using Rietan-FP and Z-code program. Moreover, the local structures were investigated using the synchrotron X-ray total scattering data (BL04B2, SPring-8) and neutron total scattering data (NOVA, J-PARC). The data was analyzed with the Pair Distribution Function method (PDF). These results were compared with electron beam diffraction image by TEM. RESULTS AND DISCUSSION From the powder X-ray diffractions, it was found that a product can be attributed to a single phase of the layered structure with space group of C2/m. In the cycle tests at room and high temperatures, 0.4Li2MnO3-0.6LiMn1/3Ni1/3Co1/3O2 electrode was able to deliver high capacity of over 270 mAh/g in the first discharge process within the voltages of 2.5 V and 4.8 V vs. Li/Li+ at the high temperature, and the capacity was higher than the value at room temperature. We prepared some electrodes with the different discharge depths in first discharge cycle, and measured the synchrotron X-ray diffraction (BL04B2, Spring-8). From the result of the measurement data (Fig. 1), it was suggested that the local structure was affected by the operating temperature. Therefore, to examine the detailed crystal structure, we determined the average structure models by Rietveld technique. As a result, it was found that the ordering of the transition metal might be changed in first charge-discharge process by the operating temperature and Ni cation mixing might be increased at high temperature. We also investigated the cation-distribution uniformity within the particle by STEM/EDS, and confirmed that there were not generally large variation in the cation distribution. In addition, we constructed local structure models by extending the refined unit cell, and then analyzed the local structure by PDF technique.Although the cathode kept the layered structure regardless of the operating temperature, the difference of the distortion parameters, σ2, of Ni-O6 octahedral of transition metal layer and Li layer was small at high temperature. It may be one of the reasons for high discharge capacity at high temperature. However, a different trend was seen in the electron beam diffraction image: that is, there was a possibility that a part of the structure was transformed to the spinel, especially around the edge of the particle at high temperature. On the other hand, at room temperature, we could observe a modulated structure of the layered structure with space group of C2/m in the electron diffraction image, which corresponded to the PDF fitting result. Reference 1) Y. Idemoto, R. Kawai, N. Ishida, N. Kitamura, J. Power Sources, 273, 1023 (2015). 2) Y. Idemoto, R. Yamamoto, N. Ishida, N. Kitamura, Electrochim. Acta, 153, 399 (2015) Figure 1

Pressure and Temperature Dependence of Local Structure and Electronic Structure of Orthorhombic DyMnO3

The dependence on pressure and temperature of the local structure and electronic state for MnO6 octahedra in orthorhombic DyMnO3 is investigated with Raman scattering, lifetime-broadening-suppressed x-ray absorption spectra and x-ray emission spectra. With increasing pressure, all Raman-active modes of DyMnO3 persist and shift monotonically to greater wavenumbers, implying that no structural change occurs under an external pressure up to \({\sim}32\) GPa. The spectral weight of the white line in Mn \(K\)-edge x-ray absorption spectra of DyMnO3 increased progressively, whereas the pre-edge feature gradually shifted to smaller energy upon increasing pressure. The Jahn–Teller distortion of MnO6 octahedra in DyMnO3 is progressively diminished with increasing pressure. The energy of the pre-edge line increased upon cooling from 300 to 10 K, in contrast with the pressure effect. A gradual breakdown of the high-spin state for MnO6 octahedra in DyMnO3 is observed. This work provides insight into structural modifi...

Temperature dependence of local structure in SrZr0.9Yb0.1O3-d

Temperature dependence of local structure in SrZr0.9Yb0.1O3-d

Temperature dependence of local structure of Ge impurity in Si (001) thin film probed by multiple-scattering XAFS

The local structure of dilute Ge impurity in Si thin film has been studied by grazing incidence fluorescence XAFS in the temperature region of 20 ∼ 300 K. Multiple-scattering XAFS (MS-XAFS) data analysis is used to fit the local structures around Ge atoms from the first to third coordination shells for the Ge0.006Si0.994 thin film. The results indicate that the bond length RGe–Si of the first, second and third Ge-Si shell is 2.38, 3.83 and 4.49 Å for the Ge0.006Si0.994 thin film, respectively. The RGe–Si of the first Ge–Si shell in the Ge0.006Si0.994 thin film is 0.03 larger than that of the first Si–Si shell in Si crystal, and the RGe–Si of the second and third Ge–Si shells in the Ge0.006Si0.994 thin film are almost equal to those of the corresponding Si–Si shells in Si crystal. These results imply that the local lattice deformation exists in the first nearest neighbor around Ge atoms and the lattice distortion relieves in the second and third Ge–Si shells for the Ge0.006Si0.994 thin film. Furthermore, the results show that the Debye-Waller factor σ12 of the first Ge–Si shell of the Ge0.006Si0.994 thin film increases slightly from 0.0018 to 0.0020 Å2 in the temperature region between 20 and 100 K, and rises strongly to 0.0040 Å2 at 300 K. The bond angles θGe–Si–Si at all temperatures are about 108.1°, which is close to the value of tetragonal network of 109.47°. The bond angle variation ΔθGe–Si–Si is about ±3.4°, ±3.5° and ±5.0° at the temperatures of 20, 100 and 300 K, respectively, indicating a weak tetragonal distortion around the Ge atoms in the Ge0.006Si0.994 thin film.

STUDY OF LOCAL STRUCTURE IN UNDER-DOPED La2-xSrxCuO4-y BY POLARIZED EXAFS

Temperature dependence of local structure of the CuO 2 plane has been studied by in plane polarized (E//ab) Cu K-edge extended X-ray absorption fine structure (EXAFS) experiments on an under-doped single crystal of superconducting La1.87Sr0.13CuO 4-y system with T c =32 K . The temperature dependence of the Debye–Waller factors for the Cu-O and Cu-La/Sr pairs are determined. We find that the motion of Cu and O atoms in the Cu-O bond is partially correlated at higher temperatures, going over to an uncorrelated motion at lower temperatures. The results are discussed in terms of the local lattice instabilities and stripes as reported in other systems.

Temperature Dependence of Local Structure Around Rb+and Cs+Ions in Alkali Doped Fullerenes

Rb and Cs K-edge EXAFS spectra have been measured in the temperature range from 10 to 290 K, in order to investigate the temperature dependence of local structure around Rb + and Cs + ions in various alkali doped fullerenes. It has been found that the alkali metal ions in Rb 3 C 60 , K 2 RbC 60 and Na 2 CsC 60 , occupy the center of the tetrahedral and octahedral sites of C 60 crystals. The metal ion in the octahedral site exhibits a large fluctuation in comparison with that in the tetrahedral site. The increase in σ(2) of the t-site Rb-C for Rb 3 C 60 has been observed around T∂ while the increase in σ(2) has not clearly been observed for the o-site Rb-C in K 2 RbC 60 . The local structure around Rb + ion in non-superconducting alkali doped fullerene Rb 6 C 60 has also been studied to compare with that in superconducting fullerenes.

A study of temperature dependence of local structure of Ln(III) chloranilate and bromanilate (Ln  Nd, Er) complexes by EXAFS spectroscopy

Temperature dependence of the local structure of lanthanoid(III) chloranilate and bromanilate complexes were examined by EXAFS spectroscopy. The interesting results are obtained. i.e., the coodination number changed 8, 7, 6 stepwise as temperature increased for Er(III) chloranilate complex. The same features are observed for Nd (III) complex. This behaviour explains the previous IR and TG results very well.

Temperature Dependence of Local Structure of Rb3C60 Superconductor

EXAFS studies on Rb3C60 are made as a function of temperature from 6K to 300 K. It is found that the distance rRb-c and the second, third and fourth cumulant terms of Debye-Waller factors, σ(2), σc(3) and σc(4), show the anomalous behavior near Tc.

A Study of Temperature Dependence of Local Structure of Erbium(III) Bromanilate Complex by EXAFS Spectroscopy

Abstract The local structure of erbium(III) bromanilate complex was studied by the EXAFS (extended X-ray absorption fine structure) and IR spectra of some different temperatures. From the EXAFS analyses the coordination numbers and the distances between Er(III) and oxygen atoms were recognized to change as temperature changed. IR spectra showed that the frequency of Er(III)-oxygen stretching vibration was increased with temperature. The water molecules involved play important roles in the local structures of this complex.