Search

Abstract: The conservation study of the Roman Heritage in Aosta (Italy) is aimed at understanding the causes of the superficial chromatic alteration, which has affected the Roman ruins (I sec a. C.) since the last decades. The Roman monuments are built of pudding stone, a quite irregular and inhomogeneous conglomerate of fluvial origin. The stone in place started showing superficial chromatic alteration as reddish and pinkish areas in the last fifty years only. First results show that the alteration might be due to the oxidation of ferrous ion. The iron oxides spread out in the secondary calcite, thanks to fluid circulation through porous material, and crumble the pudding stone surface. [Copyright &y& Elsevier]

Abstract: In this work we report on micro-Raman analysis on lithium niobate (LN) substrates in order to study the compositional homogeneity of the crystals and to clear up the effects of etching and polishing processes on the surface of wafers and crystals. The fact that the linewidth of some Raman modes scale with the composition of LN crystals, together with the use of a confocal microscope, allowed a three-dimensional determination of the sample stoichiometry and of the crystalline quality. This local tool can supply additional information, which can be complementary to the electro-optic coefficients, carefully measured as well in order to check functional parameters. Raman spectra from buried regions were obtained on as-grown, etched and polished crystals and wafers. The depth profile of the peak energy and the linewidth of the Raman mode at 872cm−1 indicate that mechanical processing of surfaces causes, in some cases, structural modifications till a depth of 15μm. [Copyright &y& Elsevier]

Abstract: Detailed photoconductivity measurements have been performed in nominally pure SrTiO3 in order to elucidate the effect of the antiferrodistorsive cubic–tetragonal phase transition. Small features in the photoconductivity’s temperature dependence in the phase transition region were found using low intensity interband UV or 514 nm light illumination. Such features are associated with a transformation of the defect system controlling the photoconductivity. At the same time, the temperature behavior of the photoconductivity spectral maximum reveals a rather unusual feature which is connected with changes in the absorption band edge structure in the phase transition region. [Copyright &y& Elsevier]

Abstract: Microstructural modifications were induced by femtosecond laser irradiation on lithium niobate (LN) crystals and investigated by means of optical microscopy and micro-Raman spectroscopy. Commercial z-cut congruent LN substrates were irradiated with focused ultra-short laser pulses at 810nm, generated by a Ti:Sapphire oscillator or an amplified Ti:Sapphire laser system. At the focus region, either refractive index changes or material removal were observed by varying the irradiation conditions. The Raman spectrum shows in the illuminated regions the insurgence of Raman modes, which are forbidden in the used-scattering geometry. This phenomenon increases gradually by approaching the ablation region, where niobium oxides and Li-deficient surfaces are formed for the low energy and high repetition rate pulses from the Ti:Sapphire oscillator. Amorphous surfaces are obtained for high energy and low repetition rate pulses from the amplified Ti:Sapphire laser system. [Copyright &y& Elsevier]

Abstract: The γ-ray-induced grafting of polybutadiene oligomers onto precipitated silica was investigated in the dose range up to 200kGy by THA, FTIR, CP/MAS NMR, TEM microscopy, inverse gas chromatography (IGC), with determination of the grafting yields and the characterization of the modified silica with respect to surface energy, SiO2–oligomer interaction and morphology. The matrix EPR spectroscopy was employed for investigating the nature of the intermediate radicals in the grafting mechanism. [Copyright &y& Elsevier]

Abstract: Ferromagnetic Co:TiO2 thin films have been grown by Co evaporation on a sputtered TiO2(100) surface. Annealing in ultra-high vacuum induced a recovery of the oxygen stoichiometry and consequently the oxidation of titanium to Ti4+ at the surface, while oxidation of the Co layer has not been observed. Room temperature ferromagnetism is observed, along with a paramagnetic contribution. Differences of the hysteresis cycle with respect to that usually observed for bulk cobalt are ascribed both to the reduced film thickness and to the disorder induced by the sputtering treatment. [Copyright &y& Elsevier]

An unusual sequence of phase transitions (PT) and reentrant dipole glass-like phase formation at low temperatures was found recently in KTaO3 weakly doped with Li and Nb (K0.9986Li0.0014Ta0.976Nb0.024O3) [Phys. Rev. B 63 (2001) 172]. We report on detailed low frequency (100 Hz–1 MHz) permittivity and Raman light scattering studies of similar composition, but without Li admixture, KTa1-xNbxO3 with x=0.018 (KTN1.8). The aim of the study is to answer the question if the reentrant dipole glass-like phase exists in KTN1.8 and what is the microscopic origin of this phase. A detailed study of the sharp low-temperature PT observed at TC∼27 K revealed properties inherent to the reentrant glass-type state at lower temperatures. The substitution of Nb for Ta influences the TO1 soft lattice mode and leads to PT with the long-range ferroelectric ordering. A crossover to an order-disorder polar microregion dynamics with a non-standard #x03B5;′(T) behaviour and dipole glass-like formation were found below TC (at ∼15 K), which is attributed to the randomness of the Nb distribution. A crossover to the long-range order was found under a dc bias field. [Copyright &y& Elsevier]

The formation of barium hexaferrite, BaFe12O19, from a 1:6 molar ratio mixture of barium oxalate and iron oxide has been investigated. Thermogravimetry (TGA), high temperature X-ray powder diffraction (HT-XRPD), differential scanning calorimetry (DSC) and micro-Raman spectroscopy have been used to determine the effect of mechanical activation on the solid state reactions occurring during heating. The resulting magnetic properties were investigated measuring hysteresis loops. For the activated mixtures, the mass loss is over at ≈600 °C i.e. well below the temperature where BaCO3 spontaneous decomposition is complete (T>850 °C). Such a noticeable temperature lowering is a consequence of the high energy milling enhancing the formation of BaFe2O4. After heating the milled mixture to 850 °C, BaFe12O19 was rapidly formed from the BaFe2O4 and residual Fe2O3. Starting from an unmilled mixture, only minor amounts of BaFe12O19 were formed by heating to 850 °C. The favourable formation of barium hexaferrite, when starting from milled powders, has been confirmed by micro-Raman spectroscopy. The powder from the activated sample was shown to have far better magnetic properties than the unactivated sample. [ABSTRACT FROM AUTHOR]

The Y2BaNiO5 compound, prototypical Haldane system characterized by anti-ferromagnetic behaviour, and its analogs Y2BaNi1−xMxO5 (M=Mg, Zn; 0.05≤x≤0.65) are investigated by X-ray diffraction and Raman spectroscopy. Structural features and phase stability ranges of the doped samples are first determined. The first order Raman scattering for the different solid solutions is measured at room temperature in order to gain new insight on lattice dynamics of the Y2BaNiO5 compound when Zn2+ or Mg2+ ions progressively substitute into Ni site.In the case of Mg substitution, the Immm space group (sg) showing NiO6 octahedra (C-6) is preserved; only a very slight lattice expansion is produced and the Raman spectrum remains practically unchanged.In Zn2+ doped samples, both Immm and Pnma sgs coexist in samples with x≤0.13, while for higher x values only the Pnma structure, having NiO5 pyramidal coordination (C-5), is present. Such a transformation is suitably observed through the changes in the Raman spectra. In particular, the lack of the inversion symmetry on the metal ion site causes the appearance of peculiar Raman modes in the frequency region between 260 and 340 cm−1. Energy and intensity of these modes are discussed taking into account the role of the doping level and of the phase abundances of the samples. The observed behaviour allows us to discuss a possible assignment of the modes in connection with the preferred insertion of Zn2+ in the C-5 polyhedra. [Copyright &y& Elsevier]