Hironobu Maeda

Accurate Bond Length Determination by EXAFS Method

The Extended X-ray Absorption Fine Structure (EXAFS) spectra of pure iron, pure platinum and intermetallic compound Fe 2 Zr, whose strtuctures were well krown, have been measured at lower temperatures (20 K–300 K), and the accuracy of the interatomic-distance determination has been estimated. In order to increase the accuracy in determining the values of the structural parameters, the FABM technique proposed by Teo et al. has been applied. In our results, however the accuracy of the interatomic distance under various temperature could be improved almost an order of magnitude by assuming that the inflection point on the absorption edge represents the edge position.

The state of excessively lon-exchanged copper in mordenite: formation of tetragonal hydroxy-bridged copper ion

Copper ions are exchanged in mordenite in amounts in excess of the value expected from stoichiometric considerations in the process of repeated ion-exchange of sodium ions in mordenite with copper ions. This is not the case for nickel and calcium ions used as exchanger ions. A series of experiments was designed to elucidate the state of excessively ion-exchanged copper in mordenite. DRS and EPR analyses together with XANES spectra revealed that the oxidation state of copper ion in mordenite is divalent in each exchange stage. IR, EPR, XANES and EXAFS studies provided evidence for the existence of a tetragonal hydroxy-bridged polymer of copper ions in excessively ion-exchanged mordenite; the Cu—O bond length is 1.97 A and the Cu–Cu distance is 3.05 A. This polymer species exhibited IR absorption bands near 3350 and 930 cm–1 for OH stretching and MOH bending vibrations of bridged species, respectively. The difference in exchange behaviour of divalent metal ions is interpreted in terms of the difference in the magnitude of their hydrolysis constants.

The Mean-Square Relative Displacement and Displacement Correlation Functions in Tetrahedrally and Octahedrally Coordinated ANB8-N Crystals

The Debye-Waller factors of atoms in Ge, GaN, ZnO, CdS, CdSe, CuBr, CuCl, AgI, CdO, AgBr, KBr and RbCl in order of Phillips ionicity (f i) are studied by both EXAFS and the diffraction method. The displacement correlation functions (DCF) are derived from the mean-square relative displacement (MSRD) and the mean-square displacement (MSD) at room temperature. The magnitudes of the MSRD and MSD correlate well with the coordination number and ionicity. The MSRDs of tetrahedrally coordinated compounds show a gradual approach to those of octahedrally coordinated ones as the ionicity increases. A divergent-like curve toward f i=0.785 is observed for the MSD in the tetrahedrally coordinated compounds, which indicates the behavior of lattice instability. In the tetrahedrally coordinated covalent materials, the MSRD is nearly half the MSD cation or MSD anion and the ratio of the DCF to MSD given by 2DCF/(MSD cation+MSD anion) is about 80%. In the octahedrally coordinated ionic materials, the MSRD is comparable to the MSD and 2DCF/(MSD cation+MSD anion)=50%. An anharmonic contribution to the Debye-Waller factor determined by EXAFS appears pronouncedly when the magnitude of σ(2) is greater than 0.01 A2. The MSRD for the tetrahedrally coordinated compounds of silver and copper halides obeys the systematic ionicity dependence and pronounced specificity of anharmonicity was not observed: AgI, CuBr and CuCl of which the high-temperature forms are typical superionic conductors have a broad interatomic potential which is similar to that of the octahedrally coordinated compounds and strongly correlated displacement in thermal vibration between cation and anion.

EXAFS and neutron diffraction studies of local and average structures for YBa2Cu2.8Zn0.2O7−δ

Abstract The local and average structures of YBa 2 Cu 3 O 7−δ doped with Zn were studied by the combined use of extended X-ray absorption fine structure (EXAFS) analysis and Rietveld refinement of neutron powder diffraction data. X-ray absorption measurements at the Zn K-edge indicate that the Zn atom has four short (2.02 A) and one long (2.25 A) Zn-O bonds. The chemical environments of the Cu and Y atoms agree well with the structure determined by neutron diffraction. The mechanism of T c of depression accompanying Zn doping is discussed.

Development of a Laboratory EXAFS Facility and Its Application to Amorphous GeSe Semiconductors

A new type of X-ray spectrometer and detector system for performing extended X-ray absorption fine structure measurements in the laboratory has been developed. The spectrometer is equipped with an asymmetrically-cut flat crystal monochromator in the magnifying mode and converts the energy dispersion to position dispersion using a position-sensitive detector. Two advantages of this apparatus are that it has no moving parts and that it makes the X-ray path shorter. The application of the spectrometer to amorphous GeSe semiconductors is also presented to show its usefulness.

Temperature dependence of the exafs spectrum in YBa2Cu3O7−δ compounds

Abstract The temperature dependence of the extended X-ray absorption fine structure (EXAFS) is studied in the high T c superconductors, YBa 2 Cu 3 O 7− δ . The measurements were done at the Cu K-edge for samples of two orthorhombic phases ( T c ≈90 K and ≈58 K, respectively) and a nonsuperconducting tetragonal phase. Interatomic distances and mean square relative displacements σ 2 for Cu-O bonds are determined by the least squares refinement. The results indicate that values of σ 2 increase near T c for both the orthorhombic samples. It is concluded that this anomalous behavior related to T c is caused by an anomalous vibration of oxygen atoms in the Ba-O layer. Changes in the Cu-O distances from 300 to 20 K are not found.

Apical Cu-O bond in YBa2Cu3O7−δ superconductors by XAFS

Abstract Careful Cu K-edge X-ray absorption fine structure (XAFS) measurements on oriented powders of YBa 2 Cu 3 O 7− δ were made as a function of temperature, of δ, T c , method of preparation and grain size. The XAFS data were analyzed for the Cu (1)- O (4) apical bond distance distribution. All samples indicated evidence for a double-peak in the distribution with a splitting of 0.1A, with the clearest case for the δ ≈ 0 samples occuring for the T c = 89K 20 μ m grain samples and the least clear one for the 5μm grain T c = 93K sample. There is no significant temperature dependence of the splitting. Because the XAFS evidence for the splitting depends on a small signal, it is prudent to verify its existence by other independent measurements.

In situ sample cell for EXAFS measurements on materials treated at elevated temperatures in vacuo

We describe the design and utilization of a simple in situ cell for extended x‐ray absorption fine structure measurements on catalysts which are treated at elevated temperatures in vacuo. Such a special apparatus permits observation of spectral changes of a sample continuously under in situ condition.

EXAFS Study on the Local Structure of a New High-Tc Superconductor in the Bi-Sr-Ca-Cu-O System

X-ray absorption spectroscopy has been used to study a new superconducting phase in the Bi-Ca-Sr-Cu-O system. X-ray absorption measurements at the Cu K-edge indicate that a copper ion is surrounded by five coordinated oxygen atoms, and that the Cu-O distance is 1.91 A. Measurements at the Bi LIII-edge give the interatomic distance between bismuth and oxygen as 2.10 A.

Local structure of (Ca, Sr)2 (Mg, Co, Zn) Si2O7 melilite solid-solution with modulated structure

The local structure around Co, Zn and Sr atoms in incommensurately modulated, melilite-type X2T1 T22O7 (X=Ca and Sr, T1=Mg, Co and Zn, T2=Si) solid-solutions has been investigated by EXAFS analyses. The modulated structure was confirmed in Ca2-xSrxCoSi2O7 solid-solutions with X=0.0 to 0.6 and for both Ca2Mg1-YCoYSi2O7 and Ca2Mg1-YZnYSi2O7 solid-solutions over the whole compositional range at room temperature.The actual bond-distances determined by the EXAFS method for the T1 site (Co-, Zn-O) in the modulated structure are longer than the mean bond-distances obtained from the X-ray diffraction method. This is attributable to the libration of the T1 tetrahedra. In the Ca1-XSrXCoSi2O7 solid-solution both the Sr-O and Co-O distances by the EXAFS method for the X-site increase from Ca end-member to Sr end-member. These increases are respectively 0.8% and 0.6%. This means the local expansions of the tetrahedral sheets and of the XO polyhedra are well matched. In the modulated Ca2Co1-YMgYSi2O7 and Ca2Zn1-YMgYSi2O7 solid-solutions, the actual Co-O and Zn-O distances for the T1-sites are nearly constant in the whole compositional range. The compositional variations of the local structure around the cations in the solid-solution are different for the X and T1 sites.It is concluded that the local geometric restriction for the size of substituted cation in X site is larger than that in T1 site. The dimension of the tetrahedral sheet puts restriction on the size of the cations situated at the interlayer X sites. In other words, the different behavior of the local geometric restriction between the X and T1 sites is an important feature of the melilite structure and is also related to the modulated structure.