E. Cazzanelli

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...

Characterization of PEO-lithium triflate polymer electrolytes: Conductivity, DSC and Raman Investigations

Polymeric membranes to be used as electrolytic separators in lithium batteries were prepared from Poly Ethylene Oxide (PEO) and Lithium Triflate salt (LiTf) via a solventfree procedure. Several membranes, having different PEO/LiTf molar ratios, were characterized by Electrochemical Impedance Spectroscopy (EIS) to obtain their ionic conductivity. Most of the compositions exhibit an Arrhenius-like behaviour with two different activation energies above and below 60 °C. Only the sample having the highest LiTf concentration showed a non-Arrhenius trend. Differential Scanning Calorimetry (DSC) study was performed to determine the various phases present in the system. The degree of association of the mobile ions changes vs. composition and temperature was investigated by Raman spectroscopy, a powerful technique which allows to follow these changes in details by studying the spectral parameters of the SO3 stretching vibration of triflate ions. These data were correlated with the conductivity and thermodynamic data.

Raman and X-ray diffraction study of boehmite gels and their transformation to α- or β-alumina

Abstract The structural evolution of gel-derived boehmite and alkali-doped boehmite samples, undergoing different heat treatments up to 1200°C, was followed by both X-ray diffraction and Raman scattering measurements, collected at room temperature. Our observations reveal the presence of different transition alumina phases at intermediate temperatures, before formation at the highest temperatures of stable α-alumina or β + β″-alumina, for alkali-loaded Al2O3. In particular, Raman data from samples treated at 400°C are consistent with a phase change from pseudo-boehmite to the γ-alumina structure, while the dehydroxylation process is not yet completed. The process appears independent of the presence of alkali: identical evolution was presented up to 800°C by all our samples. After final annealing at 1200°C the presence of alkali induces the formation of stable β + β″-alumina phases whereas, in the absence of alkali, a phase identified as Θ-Al2O3 is observed at 1000°C and, finally, stable α-Al2O3 is obtained at 1200°C.

Raman spectroscopic investigations of Li-intercalated V2O5 xerogel

Abstract Xerogel films of vanadium oxide, obtained via spin-coating method, have been analyzed by a micro Raman probe. A comparison has been made among xerogel samples coated at an angular velocity of 1500 rpm (water molar concentration 1.8), the same compounds after vacuum dehydration (water molar concentration 0.5) and the dehydrated gels intercalated with different amounts of Li+ ion. Only minor spectral changes are observed in the xerogel films after the partial dehydration. On the contrary, significant spectral modifications occur after electrochemical intercalation of Li+ ion, even for relatively low concentration (0.15 molar fraction). In fact, the strongest Raman mode of the oxide, occurring at 170 cm−1, disappears after intercalation. For increasing content of Li+ (up to 0.4 molar fraction) new Raman bands occur between 800 and 950 cm−1.

Spectroscopic investigations of Li-intercalated V2O5 polycrystalline films

Abstract Polycrystalline V 2 O 5 films (0.15 microm thick) have been electrochemically intercalated with Li + at constant current, so that to obtain a set of different cation concentrations. The open circuit potential and the optical transmittance of the Li x V 2 O 5 films were followed as a function of the lithium charge. The electrochemical measurements show a reversible Li + intercalation for x ≦1, in agreement with previous observations on pellettized V 2 O 5 powder electrodes. Some compositions ( x ∼0, 0.3, 1) have been also characterized by both absorption and micro-Raman spectroscopy, which appear to be very sensitive to the Li + content. Raman and absorption spectra seem to indicate a non complete recovery of the original electronic configuration and of the structure in the material after the inserted Li + has been fully extracted via a reverse electrochemical reaction.

Color centres and polymorphism in pure WO3 and mixed (1−x)WO3−y·xReO2 powders

The relationship between structural transformations and colour centres creation is discussed for deeply coloured hydrogen tungsten bronzes and for pure WO3 powders, acquiring less intense colour after mechanical treatments of variable duration. A comparative study on coloration is made also for mixed compounds (1−x)WO3−y·xReO2, where an evidence of a resonance effect for a particular Raman band at 970 cm−1, attributed to the color centres, is observed. Besides, it is found that even moderate milling treatments result in a quite different structural evolution of tungsten trioxide upon cooling.

Lithium ion distribution and motion in β‐alumina: An NMR and Raman investigation

NMR and Raman evidence is presented to show that, above room temperature, a change occurs in the distribution of lithium ions in Li+‐containing β‐aluminas. We interpret the results in terms of a model in which, below room temperature, lithium ions reside predominantly in positions off the center of the conduction plane (’’out‐of‐plane’’ positions). As the temperature increases, they spend an increasing fraction of their time in the middle of the conduction plane (’’in‐plane’’ positions). It is then possible, from the experimental data, to follow temperature changes of the in‐plane ion fraction and to estimate the average energy difference between in‐plane and out‐of‐plane Li+ configurations. A discussion of the Li+ dynamics is then presented, which is based upon the 7Li NMR spin‐lattice relaxation and spectral data, as well as upon Raman data.

Photo-induced transformations in pyrochlore and trirutile phases of HTaWO6

Abstract The transformations of the two known solid phases of HTaWO 6 , pyrochlore and trirutile, under visible light laser irradiation, have been investigated using Raman spectroscopy. Pyrochlore specimens show a transition to a trirutile phase after strong irradiation in air. The same effect is also obtained by heating up to 700°C. A more moderate irradiation of pyrochlore samples in an oxygen-free environment produces a different transformation to a phase similar to Ta 2 O 5 . The trirutile phase always decomposes, even under weak light exposure, into two compounds: WO 3 and another material also similar to Ta 2 O 5 .

Raman spectroscopy of HTaMO6·xH2O (M=W and Te) pyrochlores and their potassium precursors

Abstract A room temperature Raman study has been performed on polycrystalline samples of HTaWO 6 · x H 2 O and HTaTeO 6 · x H 2 O, protonic conductors, and, for comparison, on their respective potassium precursors. The hydrated and dehydrated forms of all four compounds, as well as the deuterated Ta-W pyrochlore, have been investigated with reference to host structure vibrations, which fall mainly below 1000 cm -1 , and to internal modes of H 2 O or D 2 O molecules. A comparative analysis of all these vibrations, in particular the OH stretching mode, indicates that a minimum interaction occurs between water and other species in HTaWO 6 ·H 2 O. These spectroscopic findings would exclude a direct role of the water molecule in the proton diffusion mechanism, via the formation of H 3 O + species.

Raman study and electrical conductivity of HTaMO6·H2O (M=W and Te)

Abstract Two H-pyrochlore compounds were prepared by ceramic synthesis of K + salts and H + ion exchange. A thorough electrical characterization by complex impedance spectroscopy was performed, as well as preliminary Raman investigation through all the frequency range.