A. T. Shuvaev

Anharmonic pair potential study in face-centred-cubic structure metals

Temperature dependencies of Ni, Cu and Mo metals EXAFS spectra were studied in order to determine the anharmonic pair potential. The potential parameters for metals with cubic structure - Ni, Cu, Mo as well as for Pb (Stern?E?A et al 1991 Phys. Rev. B 43 8850), Au, Ag (Newville?M and Stern?E?A http://krazy.phys.washington.edu/paper/ag-au.html) - obtained earlier, were analysed to find correlations with other physical characteristics. It was found that a, b potential parameter values correlate with cohesive energy and interatomic distance for face-centred-cubic structure metals. Obtained potential parameter values were used to determine thermodynamics parameters, including the linear coefficient of thermal expansion, the Debye temperature, the bulk modulus and the Grunesien parameter.

Structure and dielectric properties of the Aurivillius phase Bi2.25Ca0.5Na0.25Nb2O9

A new layered perovskite-like bismuth-oxide compound with the Bi2.25Ca0.5Na0.25Nb2O9 stoichiometry (Aurivillius phase) has been synthesized in polycrystalline form. As shown by powder x-ray diffraction, Bi2.25Ca0.5Na0.25Nb2O9 has an orthorhombic structure (sp. gr. A21am, no. 36) with unit-cell parameters a = 5.4845 Å, b = 5.4549 Å, and c = 24.9195 Å. Rietveld analysis has been used to determine the atomic positional parameters in its unit cell and the position of the niobium ions. Nb5+ has been shown to be displaced from the center position in the oxygen octahedra by ∼0.15 Å. The Nb K-edge extended x-ray absorption fine structure (EXAFS) spectrum of the new compound has been measured in the range 295 to 960 K. The 295-K EXAFS data have been used to evaluate the Nb-O bond lengths in the octahedra and the corresponding Debye-Waller factors. The bond lengths thus determined agree well with those inferred from powder x-ray data. With increasing temperature, the short Nb-O bonds become longer, while the long bonds become shorter. The dielectric properties of the new compound have been studied between room temperature and 1200 K at frequencies from 1 to 50 kHz. Bi2.25Ca0.5Na0.25Nb2O9 is shown to be a ferroelectric with a Curie temperature of 972 K.

Computational method of X-ray harmonic elimination in EXAFS treating

Abstract The computational method of EXAFS amplitude correction by harmonic elimination has been worked out. In conventional absorption measurements the major source of errors is the transition of harmonics by the monochromator. In this paper we present the effective correction method based on the measurements of the intensity of the X-ray radiation penetrated through calibrated metal foils both without and together with investigated samples. As an advantage of the method one may emphasize a fact that it does not require any additional data for investigated samples such as chemical composition, density, table absorption coefficients needed for existing methods.

EXAFS study of platinum coordination in compounds synthesized by cooperative evaporation of carbon and platinum in vacuum

Abstract A series of platinum compounds has been synthesized by high-temperature carbon-platinum codeposition in vacuum. The concentration of platinum atoms in the compounds ranges from 0.01 to 0.4 wt%. For the compound with 0.01% Pt concentration, the absence of the PtPt bond has been revealed and the PtC bond length and coordination number have been determined. In the remaining samples platinum atoms form metal clusters with diameter ranging from 10 to 19 A which is revealed by the EXAFS spectra.

Structure and dielectric properties of Bi5Sr(TiNb3)1 − xBxO18 (0 < x ≤ 0.25; B = Sb, V, Re) layered perovskite-like solid solutions

We have synthesized Bi5Sr(TiNb3)1 − xBxO18 (0 ≤ x ≤ 0.25) layered perovskite-like solid solutions doped with heterovalent Sb, V, and Re cations on the B site of the perovskite layer. All of the compounds are phase-pure as determined by X-ray diffraction, have Aurivillius type structures, and are close in orthorhombic cell parameters (sp. gr. A21am). The temperature dependences of their relative dielectric permittivity ɛ/ɛ0 at different frequencies show prominent high-temperature maxima corresponding to the Curie temperature of these compounds (500 to ≃690°C), which strongly depends on the concentration and nature of the B-site cations.

Structure and dielectric properties of Bi3−xNdxTi1.5W0.5O9 (0.25 ≤ x ≤ 0.5) layered oxides

A new series of polycrystalline Bi3 − xNdxTi1.5W0.5O9 (0.25 ≤ x ≤ 0.5) layered perovskite-like oxides were synthesized as single phases. The analysis of the X-ray powder diffraction data showed that all compounds crystallize in the polar orthorhombic structure of Aurivillius phase type with the space group A21am.The temperature dependences of the dielectric properties of the compounds obtained were investigated in the frequency range from 1 to 1000 kHz. For all compounds the temperature dependences of the relative permittivity ɛ/ɛ0(t) contain high-temperature maxima corresponding to the Curie temperature (tc). The latter decreased from 630 to 530°C and ɛ/ɛ0(tc) decreased by a factor of nearly 3 with an increase in the neodymium amount by a factor of 2. The found activation energies Ea of charge carriers for all compounds are close (∼0.76 eV) and independent of neodymium amount.

Temperature-dependent EXAFS-study of tungsten bronze Na2Sr4Nb10O30 and boracite Fe3B7O13Br ferroelectrics

Abstract Local structure of Na2Sr4Nb10O30 and Fe3B7O13Brvhave been studied by means of Nb and Fe K-edges EXAFS spectroscopy at temperatures from 20°C to 500°C. It has been found, that Nb atoms in Na2Sr4Nb10O30 are shifted from the center of the oxygen octahedra (by about 0.085 A) and their displacements exist below and above the phase transition point. Thus it appears that the phase transition is of the order-disorder type. Temperature dependences of Debye-Waller factor (DWF) for FeO and FeBr bonds and interatomic distances (R) in Fe3B7O13Br have pumps in the vicinity of both phase transition points. This indicates the phase transition to be of the displacive type.

Thermal conversions of dodecacarbonyltriiron on the surface of titanium and molybdenum disulfides

AbstractA study has been made of the influence of the sulfide substrate on the rate and direction of thermal decomposition, in an argon atmosphere, of Fe3(CO)12 deposited on the surface of crystalline MoS2, TiS2, and Co0.5TiS2 in amounts K=0.5−10 monolayers. It has been shown that the decomposition rate increases in the series MoS2<Co0.5TiS2<TiS2 and increases with decreasing K. The relationship between the electronic structure of the sulfide and the decomposition of Fe3(CO)12 has been examined. On the basis of

Laboratory diffractometer-based XAFS spectrometer

FeK_{\beta _5 }