J. Purans

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.

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.

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.

Li intercalation in transparent Ti-Ce oxide films: Energetics and ion dynamics

Films of Ti dioxide, mixed Ti–Ce oxide, and Ce dioxide were produced by reactive dc magnetron sputtering. Electrochemical lithiation was probed by chronopotentiometry, cyclic voltammetry together with optical transmittance recording, and impedance spectroscopy. Evidence was found for inserted electrons being accommodated in Ce 4f states; this contention was supported by preliminary results from x-ray absorption fine-structure spectroscopy. These electrons do not produce luminous electrochromism. The variation of the chemical diffusion coefficient of Li, with film composition and Li content, was also studied.

X-ray absorption spectroscopy study of local structural changes in a-WO3 under colouration

X-ray absorption spectroscopy has been applied to the study of local structural changes in amorphous tungsten trioxide thin films under colouration in an acid aqueous solution. It has been found that the formation of W5+ coloured centres is accompanied by the appearance of local deformations around them. It is shown that this effect can be associated with the formation of small-radius polarons.

X-ray excited optical luminescence detection by scanning near-field optical microscope: A new tool for nanoscience

Investigations of complex nanostructured materials used in modern technologies require special experimental techniques able to provide information on the structure and electronic properties of materials with a spatial resolution down to the nanometer scale. We tried to address these needs through the combination of x-ray absorption spectroscopy (XAS) using synchrotron radiation microbeams with scanning near-field optical microscopy (SNOM) detection of the x-ray excited optical luminescence (XEOL) signal. This new instrumentation offers the possibility to carry out a selective structural analysis of the sample surface with the subwavelength spatial resolution determined by the SNOM probe aperture. In addition, the apex of the optical fiber plays the role of a topographic probe, and chemical and topographic mappings can be simultaneously recorded. Our working XAS-SNOM prototype is based on a quartz tuning-fork head mounted on a high stability nanopositioning system; a coated optical fiber tip, operating as a probe in shear-force mode; a detection system coupled with the microscope head control system; and a dedicated software/hardware setup for synchronization of the XEOL signal detection with the synchrotron beamline acquisition system. We illustrate the possibility to obtain an element-specific contrast and to perform nano-XAS experiments by detecting the Zn K and W L(3) absorption edges in luminescent ZnO and mixed ZnWO(4)-ZnO nanostructured thin films.

X-ray absorption study of the electronic structure of tungsten and molybdenum oxides on the O K-edge

Magnetron sputtered amorphous thin films a-WO3, a-MoO3 and doped a-WO3:Ir have been studied by X-ray absorption spectroscopy on the oxygen K-edge in comparison with crystalline oxides as monoclinic m-WO3, orthorhombic α-MoO3, cubic Na0.6WO3, layered-type hexagonal h-WO3 and WO3·H2O, having variable electronic and atomic structure. The changes in the XANES ranging 10–15 eV above the absorption edge are interpreted based on the known band-structure calculations. The high-energy features are related to the multiple-scattering processes (EXAFS) at the nearest atoms.