Oxygen K-edge X-ray Absorption Spectroscopy Research Articles

Cluster dynamical mean-field calculations for TiOCl

Based on a combination of cluster dynamical mean field theory (DMFT) and density functional calculations, we calculated the angle-integrated spectral density in the layered s=1/2 quantum magnet TiOCl. The agreement with recent photoemission and oxygen K-edge x-ray absorption spectroscopy experiments is found to be good. The improvement achieved with this calculation with respect to previous single-site DMFT calculations is an indication of the correlated nature and low-dimensionality of TiOCl.

Soft X‐ray Absorption Near Edge Structure Analysis on Lithium Manganese Oxide Prepared by Microwave Heating

Microwave heating, which is recognized as a time‐saving and an energy‐efficient process, was used to synthesize manganese oxide from various starting materials. LiOH and MnO2 (electrochemically prepared manganese dioxide, EMD) were used as starting materials; ε‐MnO2‐type lithium manganese oxide that showed excellent cycle ability was the result of 3 min of microwave heating. A change in electronic structure was investigated using manganese L‐edge and oxygen K‐edge X‐ray absorption spectroscopy (XAS). The XAS results indicated that the oxidation‐reduction reaction took place in the manganese 3d and in the oxygen 2p orbitals during the charge‐discharge process and that Jahn‐Teller distortion occurred in the reduced samples.

Microscopy and Spectroscopy of Lithium Nickel Oxide-Based Particles Used in High Power Lithium-Ion Cells

Structural and electronic investigations were conducted on lithium nickel oxide-based particles used in positive electrodes of 18650-type high-power Li-ion cells. K-edge X-ray absorption spectroscopy (XAS) revealed trivalent Ni and Co ions in the bulk powder used to prepare the high power electrode laminates. Using oxygen K-edge XAS, high resolution electron microscopy, nanoprobe diffraction, and electron energy-loss spectroscopy, we identified a thick modified layer on the surface of the oxide particles, which results from the loss of Ni and Li ordering in the layered structure. This structural change was accompanied by oxygen loss and a lowering of the Ni- and Co-oxidation states in the surface layer. Growth of this surface layer may contribute to the impedance rise observed during accelerated aging of these Li-ion cells. © 2003 The Electrochemical Society. All rights reserved.

Changes in electronic structure of the electrochemically Li-ion deintercalated LiMn 2O 4 system investigated by soft X-ray absorption spectroscopy

The electronic structure of the electrochemically Li-ion deintercalated Li 1− x Mn 2O 4 system has been investigated for various Li compositions using soft X-ray absorption spectroscopy (XAS) for oxygen K-edge and Mn L II,III-edge. To get reasonably good XAS spectra for the electrochemically Li-ion deintercalated Li 1− x Mn 2O 4 system, we made a binder free LiMn 2O 4 film electrode using electrostatic spray deposition (ESD) technique. From the Mn L II,III-edge XAS, it is found that the peak shifts to higher energy with the Li-ion deintercalation and this is due to the increase of the average oxidation state of the Mn ion. From the oxygen K-edge XAS, the Li-ion deintercalation gives rise to a gradual increase in pre-edge peak intensity but mostly does not effect its peak shape.

Oxygen Contribution on Li-Ion Intercalation−Deintercalation in LiCoO2 Investigated by O K-Edge and Co L-Edge X-ray Absorption Spectroscopy

To investigate the electronic structure of the electrochemically Li-ion deintercalated Li 1 - x CoO 2 system, soft X-ray absorption spectroscopy (XAS) for the oxygen K-edge and the Co L I I , I I I -edge has been carried out intensively with compositional x value variation, compared with Co K-edge X-ray absorption near edge structure (XANES) spectroscopy. To get reasonably good XAS spectra for the electrochemically Li-ion deintercalated Li 1 - x CoO 2 system, we made a binder-free LiCoO 2 film electrode using the electrostatic spray deposition (ESD) technique. The oxygen K-edge XAS for Li 1 - x CoO 2 shows more effective spectral change with respect to Li-ion content than the Co L I I , I I I -edge XAS. The dependence of the absorption peak feature on the Li content is described in terms of the ground state of the Co and O atoms, showing the systematic variation of the hole-state site distribution between Co and oxygen atoms. From the Co L I I , I I I -edge XAS, it is found that the broad peak shift to higher energy with the Li-ion deintercalation is due to rehybridization between Co and O atoms under the local structural distortion of CoO 6 octahedra, which is also confirmed by the formation of two additional absorption peaks below the threshold energy corresponding to the oxygen 2p hole state hybridized with the 3d orbital of Co ion in the distorted CoO 6 octahedral site. In the O K-edge XAS spectra for the deintercalated Li 1 - x CoO 2 film, the shoulder absorption peak in the energy region higher than the threshold energy could be assigned to the higher oxidation state of the oxygen site on Li deintercalation. From the Co L I I , I I I -edge and O K-edge XAS results for the Li-ion deintercalated Li 1 - x CoO 2 film, it is concluded that the charge compensation for the Li-ion deintercalation process could be achieved mainly in the oxygen site and Co metal atomic site simultaneously. O K-edge and Co L I I , I I I -edge XAS results for cycled LiCoO 2 film show that the capacity fading of the LiCoO 2 system is related to the decrease of Co-O bond covalency by the local structural distortion of CoO 6 octahedra remaining in the cycled LiCoO 2 .

High-energy spectroscopy of NaV 2O 5

Oxygen K-edge X-ray absorption spectroscopy of NaV 2O 5 at various temperatures both above and below T c =34 K for E∥ a and E∥ b was carried out. The spectra taken above T c showed little temperature dependence whereas significant change was observed between the spectra taken at 70 and 25 K. This change in the electronic structure through T c may be due to the phase transition from the V–O–V molecular orbital state to the charge disproportioned ground state.

Nature of empty states in YBa2Cu3O7-x.

Angle resolved oxygen K-edge x-ray absorption spectroscopy measurements were performed on single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.9}$. Measurements were carried out on in situ cleaved crystals at 20 K to minimize the loss of oxygen from the surface region of the samples. It was found that there are two distinct transitions between 526 and 531 eV. The peaks in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.9}$ show strong orientation dependence, as well as energy dispersion of 2 eV. After the sample is heated to room temperature, there is only one peak visible which shows little energy dispersion or orientation dependence. Comparison of the experimental results to a transition-state calculation of the absorption cross section shows good agreement indicating that there are holes of \ensuremath{\sigma} symmetry on all Cu-O bonds.