Takafumi Miyanaga

Quantum statistical approach to Debye-Waller factors in EXAFS, EELS and ARXPS. I: Anharmonic contribution in plane-wave approximation

Temperature dependence of EXAFS, EELFS and ARXPS is discussed in the framework of plane wave approximation, where we take the cubic and quartic anharmonicity into account. First we apply cumulant expansion whose expanded terms are written in terms of corresponding lower order moments and cumulants. Secondly we apply temperature Greens function technique systematically to calculate these moments for perfect crystals with translational symmetry. We furthermore discuss high and low temperature behavior of these cumulants, and also describe real space representation of them in comparison with widely used classical expression. This real space representation needs no translation symmetry, and we discuss the applicability of the classical local space integral formula.

Quantum statistical approach to Debye-Waller factor in EXAFS, EELS and ARXPS. II: Application to one-dimensional models

Temperature dependence of Debye-Waller factor in EXAFS, EELS and ARXPS is calculated for one-dimensional models on the basis of first principle quantum statistical method developed in the previous paper I. In contrast to diffraction Debye-Waller factors for crystal we have no need to worry about the divergence for these thermal factors in one- and two-dimensional phonon systems. We consider anharmonic Morse potential as nearest neighbor interaction. We take up to quartic term into account and estimate up to fourth order cumulants for both monatomic and diatomic chains in wide temperature range from low temperature in which quantum effect is important, to high temperature in which classical approximation works. Characteristic features of the contributions from optical and acoustic modes are discussed in the diatomic model chain. The result obtained here give us clear physical insights of those thermal factors.

Charge–discharge reaction mechanism of manganese vanadium oxide as a high capacity anode material for lithium secondary battery

Crystalline MnV2O6 with brannerite structure was synthesized by a classical solid-state reaction and its electrochemical reaction mechanism as an anode for a Li secondary battery was investigated. The reversible capacity of the MnV2O6 anode during the first cycle was about 670 A h kg−1. An irreversible structural transformation into an amorphous phase via a rock-salt type structure was observed during the first Li insertion/removal reaction by X-ray diffraction (XRD) analysis. The charge compensation and the change in the local environmental structure during the Li insertion/removal were confirmed by X-ray absorption fine structure (XAFS) of Mn and V.

Studies on the structure of molybdenum(VI) in acidic solution by xanes and exafs

Abstract X-ray absorption spectra for molybdenum(VI) in acidic solutions were measured and their XANES and EXAFS were analysed. The cationic monomer MoO22+ is present in HClO4, H2SO4 and HNO3. In HCl the coordination of chloride ions to MoO22+ was observed. Diethyl ether extracts MoO2Cl2 selectively from a HCl solution of molybdenum(VI).

Charge–discharge reaction mechanism of manganese molybdenum vanadium oxide as a high capacity anode material for Li secondary battery

The compound system, Mn1−xMo2xV2(1−x)O6 (0 ≤ x ≤ 0.4), synthesized by solid state reaction forms the brannerite structure. The electrochemical reaction mechanism as the anode for Li secondary batteries was investigated. X-ray diffraction (XRD) analysis clearly indicated that Mn0.6Mo0.8V1.2O6 anode irreversibly transformed from crystalline brannerite to amorphous phase via a rock-salt type structure during the first lithium insertion/removal reaction. The charge compensation and the change in the local environmental structure during the Li insertion/removal were confirmed by X-ray absorption fine structure (XAFS) of manganese, vanadium and molybdenum.

Quantum Statistical Approach to Debye-Waller Factors in EXAFS, EELS and ARXPS IV. Anharmonic Contribution in Spherical-Wave Expression

In this work we have developed a new approach based on Brouders Lie group formulas to include both spherical wave correction and anharmonic vibration effects, which is not limited to isotropic systems. We apply partial summation technique to get a p l a n e w a v e p a r t which does not disappear even for large k R limit, and a s p h e r i c a l w a v e p a r t which disappears for that limit. The former part has just the same cumulants as those used for plane wave EXAFS formulas, however, the latter cannot be reduced to the ordinary cumulants and is closely related to the correction factor due to thermal vibration derived by Rennert within the harmonic approximation. The plane wave part already includes spherical wave effects of photoelectron scattering. Numerical calculations for K-edge EXAFS show that s p h e r i c a l w a v e thermal factors give rise to different phase from p l a n e w a v e p a r t , which can give peak shift in Fourier transformed EXAFS spectra. The polarization dependence is als...

Semiconductor-metal transition in liquid As-Te mixtures

EXAFS, conductivity and Hall coefficient measurements have been carried out for liquid As-Te mixtures. EXAFS analysis reveals the presence of chemical disorder marked by the existence of homopolar As-As and Te-Te pairs. At high temperature around 500 ◦ C the network structure composed of threefold coordinated As atoms and twofold coordinated Te atoms is transformed into the twofold chain structure. The network-chain transformation is accompanied by the gradual semiconductor to metal transition which is demonstrated by the results for conductivity and Hall coefficient. The microscopic origin of the semiconductor to metal transition is discussed in connection with the structural modification.

Quantum Statistical Approach to Debye-Waller Factor in EXAFS, EELS and ARXPS. III. Applicability of Debye and Einstein Approximation

We investigate the availability of Debye and Einstein approximations to calculate Debye-Waller factors in EXAFS, EELS and ARXPS for one-dimensional models. These approximate results were compared with the ab initio results shown in the previous paper II. For monatomic chains the Debye model can give a good result even for third and fourth cumulants. In the case of diatomic chains, a mixed model where Debye (Einstein) approximation is used for acoustic (optical) mode give quite a good result for harmonic second order cumulant. This simple approximation, however, gives rather poor results for third and fourth order cumulants and also anharmonic correction to the second order cumulant. This result is due to the fact the phases of the eigenvectors of the dynamical matrix play an important role for the accurate estimation of those thermal factors.

Quantum Statistical Approach to Debye-Waller Factors in EXAFS, EELS and ARXPS. V. Real Space Approach to One-Dimensional Systems

In this paper thermal effects in EXAFS due to atomic vibration are studied by use of the Feynmans path integral approach. This approach can be applied to strongly anharmonic systems where the cumulant analyses break. It is closely related to the well-known classical approach which is only valid at high temperature, though it can be applied to the strongly anharmonic systems. This paper describes the results applied to one-dimensional systems with quartic anharmonic potential and double-well potential. In the former case the cumulant analyses work well even for rather strong anharmonic potential, whereas in the latter case the tunneling effects play some important role to give rise to a sharp peaks in probability density at low temperature, there the classical approximation cannot be applied but the cumulant approximation gives the reasonable results. On the other hand the classical approximation gives quite good results, but the cumulant approximation gives poor results at high temperature in the latter ...

Photoluminescence of the Dehydrated Ag-type Zeolite A Packed under Air

The photoluminescence (PL) and extended X-ray absorption fine structure (EXAFS) measurements have been carried out for the dehydrated fully Ag + exchanged zeolite A powders packed with controlled amounts of the adsorbed H 2 O molecules. The powders are designated as “air-packed 12Ag-A” and have been prepared by packing the dehydrated 12Ag-A in the glass cell under air at different temperatures between 120 and 30 °C in the cooling process after the dehydration at 500 °C for 24 h under air. The PL spectra obtained by radiation of 400 nm light have main peaks around 2.15 eV and subpeaks around 2.70 eV. The PL spectra were fitted by four Gaussians. The peak around 2.15 eV consists of the band I ∼1.94 eV and band II ∼2.10 eV, while the subpeak around 2.70 eV of the band III ∼2.50 eV and band IV ∼2.80 eV. The temperature variations of the integral intensities for the bands II and IV show maxima between 50 and 80 °C where the interaction between the zeolite framework and the adsorbed H 2 O molecules is found to ...