A. Kuzmin

Raman scattering in nanosized nickel oxide NiO

Magnetic ordering in nanosized (100 and 1500 nm) nickel oxide NiO powders, prepared by the plasma synthesis method, was studied using Raman scattering spectroscopy in a wide range of temperatures from 10 to 300 K. It was observed that the intensity of two- magnon band decreases rapidly for smaller crystallites size. This effect is attributed to a decrease of antiferromagnetic spin correlations and leads to the antiferromagnetic-to- paramagnetic phase transition .

Raman study of the phase transitions sequence in pure WO3 at high temperature and in HxWO3 with variable hydrogen content

Abstract An extensive investigation of the temperature dependence of Raman spectra has been carried out on WO 3 powders from room temperature to 800°C. In particular the orthorhombic-to-tetragonal phase transition occurring at about 740°C has been studied for the first time. The Raman active mode at 710 cm −1 of the orthorhombic phase disappears from the spectrum at temperature below the phase transition point and the Raman activity in the tetragonal phase results very low. A comparative study of hydrogenated tungsten bronzes H x WO 3 ( x ≤0.23), where the same transition sequence is driven by an increase of the proton concentration from x =0 to 0.23, reveals similar behaviour of the high frequency Raman active modes, which are better resolved due to the weaker anharmonic interactions.

EDA: EXAFS data analysis software package

Abstract The present paper describes the EXAFS data analysis software package, called EDA, originally developed by the author for IBM PC compatible computers. It consists of a set of interactive programs which allow to carry out all steps of the EXAFS data analysis procedure. There are two main differences from known packages. First, a significantly improved algorithm is used for atomic-like background removal in the EXAFS extraction procedure. Second, a model independent derivation of the radial distribution function from EXAFS, based on a maximum-entropy-like algorithm, is available.

X-ray diffraction, extended x-ray absorption fine structure and Raman spectroscopy studies of WO3 powders and (1−x)WO3−y⋅xReO2 mixtures

Pure ground tungsten trioxide WO3 and (1−x)WO3−y⋅xReO2 mixtures were studied by x-ray absorption spectroscopy, x-ray powder diffraction and Raman spectroscopy in comparison with hydrogen bronzes HxWO3 and hydrogenated calcium tungstate CaWO4:H. It was found that a grinding of pure WO3 leads to a decrease of the crystallites size and a development of the bluish coloration. The color change was found to be reversible under moderate heat treatment or after storage in oxidizing atmosphere and is attributed to the reduced W5+ ions, located at the surface of freshly ground powder. The (1−x)WO3−y⋅xReO2 mixtures were found to be composed of monoclinic/orthorhombic WO3 and orthorhombic ReO2 phases with a grain boundary containing reduced W5.7+ ions which are mainly responsible for the compound color at low rhenium ion concentrations. In both cases, the W(6−z)+ (0<z⩽1) color centers are responsible for strong optical absorption resulting in the dramatic decrease of the total Raman intensity. The structural models o...

XAS, XRD, AFM and Raman studies of nickel tungstate electrochromic thin films

Abstract Systematic studies of nanocrystalline nickel tungstate, NiWO 4 , thin films were performed by several experimental techniques such as Ni K- and W L 1,3 -edges X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, atomic force microscopy and cyclic voltammetry measurements. We found that the NiWO 4 thin films exhibit electrochromic properties similar to that of amorphous tungsten trioxide films, but show better structural stability upon multiple colouring/bleaching cycling. It was observed that a nanocrystallinity of the thin films results in strong modifications of the NiO and WO interactions, which affect both local atomic and vibrational structures.

Dehydration of the molybdenum trioxide hydrates MoO3·nH2O: in situ x-ray absorption spectroscopy study at the Mo K edge

The process of dehydration for the molybdenum trioxide hydrates, MoO3·2H2O → β-MoO3·H2O → α-MoO3, was studied for the first time by in situ Mo K-edge x-ray absorption spectroscopy. A regularization procedure was applied to reconstruct the radial distribution functions around the molybdenum at all stages of the process. Strong variations in the local environment of the molybdenum were found at both phase transitions and were attributed to the changes in direction of the off-centre Mo displacement and in the degree of symmetrization of the Mo-O bonds. Also, an increase of the Mo-O-Mo angle, leading to the appearance of strong multiple- scattering effects in the x-ray absorption fine structure, was observed at the second phase transition β-MoO3·H2O → α-MoO3. The final α-MoO3 product had nanocrystalline structure with an average grain size of about 15 A.

Study of vibrational and magnetic excitations in NicMg1−cO solid solutions by Raman spectroscopy

The Raman scattering by phonons and magnons was studied for the first time in the polycrystalline solid solutions NicMg1−cO. The experimental Raman spectrum for c = 0. 9i s similar to that of NiO and consists of six well resolved bands, whose origins are the disorder-induced one-phonon scattering (bands at 400 and 500 cm −1 ), two-phonon scattering (bands at 750, 900, and 1100 cm −1 ), and two-magnon scattering (the broad band at ∼1400 cm −1 ). We found that the dependence of the two-magnon band in solid solutions on the composition and temperature is consistent with their magnetic phase diagram. We also observed that the relative contribution of two-phonon scattering decreases strongly upon dilution with magnesium ions and disappears completely at c < 0.5. Such behaviour is explained in terms of a disorder-induced effect, which increases the probability of the one-phonon scattering processes.

EXAFS and XANES analysis of oxides at the nanoscale.

This work presents a discussion of the possibilities offered by X-ray absorption spectroscopy (XAS) to study the local structure of nanomaterials. The current state of the art for the interpretation of extended X-ray absorption fine structure (EXAFS), including an advanced approach based on the use of classical molecular dynamics, is described and exemplified in the case of NiO nanoparticles. In addition, the limits and possibilities of X-ray absorption near-edge spectroscopy (XANES) in determining several effects associated with the nanocrystalline nature of materials are also discussed in connection with the development of ZnO-based dilute magnetic semiconductors and iron oxide nanoparticles.

X-ray absorption spectroscopy study of the Ni K edge in magnetron-sputtered nickel oxide thin films

Nickel oxide, NiOx , thin films were studied by means of x-ray absorption spectroscopy at the Ni K edge. Thin films were prepared by reactive dc magnetron sputtering of a nickel target in an atmosphere that was an Ar-O2 mixture. The results of the EXAFS analysis show that the thin films have nanocrystalline structure, which can be described within the non-reconstructed grain-boundary model. Average grain sizes of 21 3 ˚ A and 34 5 ˚ A were obtained for thin films deposited in sputter atmospheres with oxygen contents of 30% and 100%, respectively. The nanocrystalline structure of the thin films results in a reduction in the number of atomic neighbours in the crystal lattice coordination shells beyond the first one. Also, increases of the structural disorder and of the intragrain lattice parameter relative to those for crystalline NiO (c-NiO) were detected for the thin films. In the XANES region, the intensity of the pre-edge peak, attributed to the dipole-allowed transitionsj1s3d 8Ci L i i!j1s3d 8Ci L i 1 i ;i D 1 ;2 (where L denotes an O 2p hole and 1s stands for a Ni 1s core hole), was found to be smaller in the thin films than in c-NiO. This result was interpreted within the Zaanen-Sawatzky-Allen model as being due to an increase of the oxygen-nickel charge-transfer energy which is accompanied by a reduction of the extent of the 3d 8Ci L i .iD 1; 2/ ground-state configuration. The findings on the local atomic and electronic structures suggest that, compared to that for c-NiO, the ionicity of the nickel-oxygen bonding increases significantly in the thin films.

Spectroscopic study of the electric field induced valence change of Fe-defect centers in SrTiO3

The electrochemical changes induced by an electric field in Fe-doped SrTiO(3) have been investigated by X-ray absorption spectroscopy (XANES and EXAFS), electron paramagnetic resonance (EPR) and Raman spectroscopy. A detailed study of the Fe dopant in the regions around the anode and cathode reveals new insights into the local structure and valence state of Fe in SrTiO(3) single crystals. The ab initio full multiple-scattering XANES calculations give an evidence of the oxygen vacancy presence in the first coordination shell of iron. Differences in the length and disorder of the Fe-O bonds as extracted from EXAFS are correlated to the unequivocal identification of the defect type by complementary spectroscopical techniques to identify the valence state of the Fe-dopant and the presence of the Fe - V(Ö) complexes. Through this combinatorial approach, novel structural information on Fe - V(Ö) complexes is provided by X-ray absorption spectroscopy, and the relation of Fe-O bond length, doping level and oxidation state in SrTi(1-x)Fe(x)O(3) is briefly discussed.