Alojz Kodre

Atomic and electronic structure of Mo6S9−xIx nanowires

Moybdenum-based subnanometre diameter nanowires are easy to synthesize and disperse, and they exhibit a variety of functional properties in which they are superior to other one-dimensional materials. However, further progress in the understanding of physical properties and the development of new and specific applications have so far been impeded by the fact that their structure was not accurately known. Here we report on a combination of systematic x-ray diffraction and extended x-ray absorption fine structure experiments, and first-principles theoretical structure calculations, which are used to determine the atomic skeletal structure of individual Mo6S9−xIx (MoSIx) nanowires, their packing arrangement within bundles and their electronic band structure. From this work we conclude that the variations in functional properties appear to arise from different stoichiometry, not skeletal structure. A supplementary data file is available from http://stacks.iop.org/0957-4484/16/1578

Adsorption of cetyltrimethylammonium bromide on carbon black from aqueous solution

Abstract The adsorption of cationic surfactant cetyltrimethylammonium bromide (CTAB) from aqueous solution onto carbon black was studied using charge-compensating polyelectrolyte titration for quantitative determination of the surfactant content in solution and X-ray powder diffraction analysis of air-dried dispersions. Sharp saturation was observed. The saturated surface concentration of the adsorbed CTAB was found to be the same in the dispersed and air-dried product. Fourier transformed infrared spectroscopy and extended X-ray absorption fine structure (EXAFS) analyses suggest -CTA+ and Br- ion sites in the dried product.

EXAFS determination of the size of Co clusters on silica.

The metallic Co catalyst for the Fischer-Tropsch reaction is prepared by reduction of Co salts impregnating microporous silica. The average size of the metallic Co clusters is determined from the average number of neighbours deduced from Co K-edge EXAFS of catalyst samples. A model EXAFS signal constructed from the scattering paths of Co metal fcc lattice with lengths up to 5 A is calibrated on a reference spectrum of Co metal foil. Catalyst spectra are interpreted with the same model expanded with variable neighbour fractions of the four nearest shells. Cluster size is obtained from comparison with the neighbour-fractions of consecutive fcc magic-number clusters.

Detailed In Situ Investigation of the Electrochemical Processes in Li2FeTiO4 Cathodes

The possible mechanism of lithium exchange from Li 2 FeTiO 4 is discussed on the basis of three different in situ structural characterization methods. The in situ X-ray diffraction (XRD) showed that the mechanism of lithium exchange was through a solid solution formation whereby the change in volume during the oxidation was 1.4%. The reversibility in terms of charge recuperation was complete; however, after the first reduction, the change in volume was still 0.4%. In situ Mossbauer spectroscopy experiment showed complete Fe(II) oxidization to Fe(III), while the reduction was not reversible. After the complete first cycle, the number of local environments in the Mossbauer spectra needed for a successful fit increased by at least two. Several local environments can be explained on the basis of extended X-ray absorption fine structure results. In the as-prepared sample, Fe cations are coordinated to three oxygen atoms at 2.03 A and another three at 2.13 A. With the oxidation of Fe, the splitting of two oxygen atoms gradually increases to 0.18 A, and the octahedra deformation gradually changes from 3 + 3 to 4 + 2. This deformation is not reversible and it can explain changes in the long-range order as observed from in situ XRD.

Extended x-ray absorption fine structure study of phase transitions in the piezoelectric perovskite K0.5Na0.5NbO3

Following an x-ray diffraction study of phase transitions of the piezoelectric perovskite K0.5Na0.5NbO3 the structural changes of the material are studied using extended x-ray absorption fine structure analysis, whereby the neighborhood of Nb atom is determined in the temperature range of monoclinic, tetragonal, and cubic phases. Within the entire range Nb atom is displaced from the center of the octahedron of its immediate oxygen neighbors, as witnessed by the splitting of Nb–O distance. The model shows high prevalence of the displacement in the (111) crystallographic direction of the simple perovskite cell. The corresponding splitting of the Nb–Nb distance is negligible. There is no observable disalignment of the linear Nb–O–Nb bonds from the ideal cubic arrangement, judging from the intensity of the focusing of the photoelectron wave on the Nb–Nb scattering path by the interposed oxygen atom. As a general result, the phase transitions are found as an effect of the long-range order, while the placement ...

Multielectron excitations in x-ray absorption spectra of Rb and Kr

In the K-edge x-ray absorption of rubidium vapour and gaseous krypton, comprehensive spectra of collective excitations down to the relative probability of 5 × 10 −5 are extracted. After removal of the asymptotic Victoreen trend, the region ∼100 eV above the edge exhibits a steep decrease of the absorption coefficient attributed to core relaxation and post-collision interaction in the Auger decay. With an exponential model for th ed ecrease,the entire spectrum of multielectron excitations is recognized as as uccession of consecutive resonant, shake-up and shake-off channels. They can be identified as coexcitations of electrons from successively deeper subshells, from 5s down to 3s. The identification of excited states is aided by a quantitative modelling of subshell contributions and by natural-width deconvolution. The valence and subvalence coexcitations are shown to follow the pattern of the lighter homologues potassium and argon.

Homogeneity of Pb(Zr,Ti)O3 thin films by chemical solution deposition: Extended x-ray absorption fine structure spectroscopy study of zirconium local environment

Sols for Pb(Zr0.53Ti0.47)O3 (PZT) thin films were prepared by 2-methoxyethanol route from lead acetate, titanium n-propoxide, and zirconium n-propoxide, the latter either unmodified or modified with acetylacetone or acetic acid in a 2∕1 molar ratio and deposited on sapphire (0001). By Zr K-edge extended x-ray absorption fine structure (EXAFS) spectroscopy, the structural changes in the Zr local environment, induced by the addition of the two modifiers, were followed from the synthesis of the PZT sol to the transition to the amorphous film. In the unmodified PZT sol segregation of Zr species occurs from the original dimers present in the Zr propoxide solution in 2-methoxyethanol. The immediate neighborhood of Zr atoms changes markedly at the transition from the sol to the amorphous film: the local structure around Zr atoms is similar to the one found in tetragonal zirconia particles. The modification of Zr propoxide with acetylacetone in 2-methoxyethanol results in Zr monomers. In PZT sol, clustering of Zr...

Atomic background and EXAFS of gaseous hydrides of Ge, As, Se and Br

K edge absorption spectra of elements from Ge to Br in gaseous hydrides were measured at the BM 29 station of ESRF with noise level as low as 2x10(-5), so far achievable only on noble gases. The dominant feature of the spectra is the atomic background similar to that of Kr and comparable to it in the quality of detail. Onto the background, a smooth weak EXAFS signal due to the hydrogen neighbors is superposed. For the very simple molecules such as these, its contribution can be calculated ab initio, with exact treatment of the effect of molecular vibrations, so that no best-fit adjustment to the experimental data is necessary. In comparison to the main constituent of the atomic background, i.e. the shake-up absorption edges, the EXAFS signal is of minor importance in HBr with a single H neighbor, but of the same order of magnitude in GeH4.

EXAFS Study of Amorphous Precursors for Pb(Zr,Ti)O3 Ceramics

In as-hydrolyzed amorphous precursors of Pb(Zr,Ti)O3 (PZT) ceramics with various Zr/Ti ratios prepared from lead acetate and transition metal n-butoxides in parent alcohol medium the local environment of constituent metal atoms has been determined by EXAFS. The local zirconium atom environment in high-Zr precursors consists of distorted octahedra of oxygen atoms with all the bonds made up of Zr–O–Zr links. For low-Zr precursors a smaller distortion of the oxygen octahedra is observed, and two Ti atoms are found in the second shell. The titanium local environment is almost independent of the Zr/Ti ratio, with five oxygen atoms in the first and two titanium atoms in the second shell of neighbors. Lead atoms contain two oxygen atoms in the first coordination shell. Pb–O–M (M = Zr, Ti) links are identified in all precursors: the former at Zr/Ti 75/25 and the latter at lower Zr/Ti ratios. Although a pronounced tendency for the homocondensation of zirconium species is found down to a Zr/Ti 50/50 ratio, heterometallic links (Pb–O–M) are determined in PZT precursors.

PZT thin films prepared from modified zirconium alkoxide

Abstract Selective modification of zirconium alkoxide by acetic acid or acetylacetone was used in solution processing of PZT Pb(Zr, Ti)O3 thin films by 2-methoxyethanol route. EXAFS results clearly show a decrease of Zr-O-Zr links in the acetic acid modified PZT sol implying a more homogeneous constituent metal distribution. No such decrease in comparison to unmodified sol is found in acetylacetone modified PZT. Higher level of homogeneity achieved in modified PZT sols evokes the perovskite crystallization in thin films and decreases the crystallization temperature on (0001) sapphire. On TiO2/Pt/TiO2/Si the modifiers affect the relative orientation of the perovskite phase.