S. Stizza

Study of amorphous and crystalline Li1+xV3O8 by FTIR, XAS and electrochemical techniques

Abstract Crystalline and amorphous Li 1+ x V 3 O 8 have been compared by electrochemical and spectroscopic investigations. In particular, the behavior of the two materials in cyclic voltammetric experiments at very low scan rate and the Li + diffusion coefficient have been studied. The cyclic voltammetry of crystalline Li 1+ x V 3 O 8 shows the existence of well defined site energies, whilst in the amorphous form the sites have a rather broad energy distribution. Higher D Li + values have been measured in amorphous Li 1+ x V 3 O 8 , this being in agreement with its higher rate capability. The different electrochemical behavior of the two forms have been explained on the basis of structural information obtained by FTIR and X-ray absorption spectroscopy (XAS). These have shown, in particular, that the V-O distances are more homogeneous in the amorphous form and that a partial amorphization occurs in the crystalline form upon Li + intercalation.

Structure of densified vitreous silica: Silicon and oxygen XANES spectra and multiple scattering calculations

The decrease of the mean Si-O-Si angle in vitreous silica upon densification from 2.20 to 2.36 gcm-3 has been followed by oxygen and silicon K-edge XANES spectroscopy. Multiple scattering calculations using clusters of two shells around the oxygen and silicon atoms, respectively, are in good agreement with experimental absorption spectra and confirm mean Si-O-Si angles between 130 and 144° for these samples, and a decrease of the mean angle with densification. The experimental spectra also exhibit features due to scattering at outer (>2) shells around the photoabsorbers.

Xanes (X-ray absorption near edge structure) of V in vanadium-iron phosphate glasses

The X-ray Absorption Near-Edge Structure (XANES) of V in vanadium-iron glasses (50P2O5 + (50−x)FeO + xV2O5) have been measured. The effective charge of V ions in glasses has been determined. At low V2O5 concentration (x ∼ 5) only V4+ with 6-fold coordination is present on the contrary a static mixed valence state (V4+, V5+) has been found at high concentrations 20⩽x⩽50. The results explain the electron hopping conductivity effects at high V2O5 concentration (x ∼ 50) involving V4+ − V5+ pairs and at low V2O5 concentration (x≅10) involving V4+ − Fe3+ pairs.

Experimental evidence for the “shake-down” peak in LIII (and LII)-xanes of light rare earth intermetallics

Abstract LIII and LII x-ray absorption near edge structure, XANES spectra of light rare earths R = La, Ce, Pr, Nd and Sm in isomorph intermetallic compounds RPd3 have been measured at the Frascati Synchrotron Radiation Facility. We report evidence for break down of one-electron interpretation of XANES in light rare earth compounds where a localized valence band [4f] is hybridized with a delocalized valence band. A “shake-down” feature in the low energy side of the 2p→ 5, ed white line in the XANES of LaPd3 and CePd3 has been found. The intensity of the “shake-down” peak decreases going from LaPd3 to heavier isomorph compounds with decreasing 4f hybridization. In NdPd3 and SmPd3 the “shake-down” is quenched. The intensity of the “shake-down” is much weaker than the corresponding shake-down satellites in the 3d core level XPS spectra (in LaPd3 the shake-down is 12% of the main one-electron white line in XANES to be compared with a shake-down satellite 90% of the core direct photoemission line in XPS). Our results show that the multi-electron excitations play a minor role in XANES.

Electroconductivity of amorphous carbon films containing silicon and tungsten

Abstract Data on the electron transport in insulating amorphous carbon films containing silicon and tungsten atoms are presented. The films were grown by plasma-assisted chemical vapour deposition. The tungsten atoms were introduced into the growing film from the hot filament. The structure of these films consists of an atomic-scale composite of carbon and silicon random networks. The carbon network is stabilized by hydrogen and the silicon network is stabilized by oxygen. Such self-stabilized C-Si amorphous structures form an ideal matrix for the introduction of metals, in particular transition metals. These metals are distributed as separate atoms or as separate disordered networks. The addition of metal atoms allows the electrical resistivity to be changed from 1014–1015 Ω cm down to 10−4 Ω cm in a controlled way. Various electron transport mechanisms and percolation phenomena are observed in the diamond-like nanocomposite. It was found that in the temperature range 150–350 K the electroconductivity is of thermo-activated Pool-Frenkel nature and is characterized by two values of activation energy, 0.32 and 0.2 eV. The observed saturation of the current vs. temperature on increasing the electric field is connected with the predominance of tunnelling effects (activationless hopping conductivity and direct tunnelling at the mobility threshold). The week magnetoresistance proportional to the square of the magnetic field can be explained by the small value of localization radius at the Fermi level.

Asymmetries in the optical properties of vacuum-deposited organic films illuminated at the substrate and non-substrate surfaces

Abstract Tetracene, pentacene and nickel phthalocyanine films were prepared by vacuum deposition onto glass substrates. Their reflection spectra on illumination of the substrate (S) and the non-substrate (NS) surfaces were measured as a function of the sample thickness d. The reflectivity R and band shape of the spectra change with d and differ when measured from the S surface and the NS surface. These changes and asymmetries are interpreted as due to four factors: (i) the difference in surface roughnness, (ii) the difference in the refraction index at the film-air interface (NS) and the film-glass interface (S), (iii) the presence of stresses in the films and (iv) the Rayleigh-type scattering of light enhanced by the contact medium. A detailed analysis indicates the first and second factors to be dominant. Previous data for asymmetries in photoconductivity and photovoltaic effects can be consistently explained in terms of roughness-induced differentiation in the energy and spatial distributions of the structural defects in the near-surface layers of such films.

Electrochemical, ZAS and FTIR study of lithium intercalation in Na1+xV3O8

Abstract Slow scan rate cyclic voltammetry, X-ray absorption (XAS) and FTIR spectroscopy have been used to study the intercalation of lithium in Na 1+ x V 3 O 8 prepared using a high temperature (HT) and a low temperature (LT) synthetic route. All techniques provide evidence for different structural evolution in the two forms as lithium is inserted. The LT form has been found to be able to intercalate more lithium than the HT form (4.5 versus 2.3 moles per unit formula). The ability of the LT form to intercalate more lithium can be related to a limited long range order that is further reduced upon lithiation. The XAS spectra reveal that, during lithium intercalation, a partial loss of long range order also occurs in the HT form. The lithium apparent diffusion coefficients in both forms have also been measured electrochemically and found to be very similar.

EXAFS Experiment on a Copper Liquid Crystalline Coordination Compound

The results on the EXAFS experiments performed on a liquid crystalline copper (II) coordination compound are reported. To the best of our knowledge these are the first results concerning a calamitic liquid crystal. The shell-by-shell data analysis, either in solids or SB mesophase, shows a copper-copper correlation at 3.85 A, a distance which roughly corresponds to the molecular thickness. These results, together with the X-ray measurements previously carried on a monocrystal, suggest a local biaxial smectic phase.

Phase transitions and undercooling in confined gallium

Abstract Phase transitions in submicrometre gallium droplets confined in epoxy resin are revealed by single-energy X-ray absorption spectroscopy using synchrotron radiation, a technique previously applied in high-temperature studies of solid and liquid metals. These unconventional measurements are accompanied by a detailed characterization of samples using scanning electron microscopy and adiabatic calorimetry measurements. Exceptional undercooling down to 150 K is confirmed by the present data. Specific heat measurements performed during the warming cycles confirmed the presence of three different solid phases for gallium, namely β-Ga, γ-Ga and δ-Ga and the absence of the α-Ga stable phase at ambient conditions.

The local electronic structure of PdO crystal and PdO catalyst supported on SiO2 and γ-Al2O3 from L3 and L1 x-ray absorption Pd edge in XANES spectra

Abstract We have studied the local electronic structure of PdO catalysts supported on characteristic inert and quasi-inert substrate from experiment on X-ray absorption near edge structure XANES using Synchrotron radiation. From the joint analysis of L 3 absorption edge and XPS core data in Pd and PdO we find that the white line in PdO XANES is an excitonic state with 0.8 eV binding energy. From the joint analysis of L 3 and L 1 edges the p-like and the d-like local unoccupied electronic states have been determined. The local structure of PdO catalysts is different from that of PdO crystal. Evidence for structural disorder in PdO catalysts and PdO-substrate interaction is reported.