Ariete Righi

Raman-spectroscopic study of lanthanide trifluorides with the β-YF3 structure

We have performed polarized Raman scattering measurements on several Czochralski-grown lanthanide trifluoride (LnF3) crystals presenting the β-YF3 structure (space group: Pnma). The phonon mode assignments for the DyF3 and LuF3 spectra are given for the first time. Besides this, the Raman spectra of TbF3, ErF3 and YbF3 complete and extend previous studies on these materials. The vibrational modes of LnF3 compounds are then correlated with those of the archetype β-YF3 crystal. The totally symmetrical fundamentals present a strong dependence on the scattering geometry, similar to what is shown by molecular crystals with weakly interacting molecules. Most of the Raman features are well behaved with the lanthanide ion substitution. Although the observed modes always involve all ions, it was possible to discern modes that are dominated by the motion of either the lanthanide or the fluorine ions. The intensity of some low frequency modes has proved to be quite dependent on the orthorhombic distortion of the quasi-hexagonal structure.

Synthesis of C60(OH)18-20 in aqueous alkaline solution under O2-atmosphere

In this work we report on an alternative synthesis of water-soluble fullerenes known as fullerols, aiming for biomedical applications. The synthesis is based on a process in which polyethylene glycol (PEG400) is used as phase-transfer catalyst between fullerene/benzene and aqueous NaOH solutions. The polyhydroxylation of the fullerenes occurs in the NaOH solution under a continuous flow of O2 to enhance the reaction yield. The resulting compound was characterized with infrared spectroscopy, nuclear magnetic resonance, thermo-gravimetric analysis and optical absorption. The formation of C60(OH)18-20 in high yields was confirmed.

Raman and infrared study of hydroxyl sites in natural uvite, fluor-uvite, magnesio-foitite, dravite and elbaite tourmalines

We present the Raman and infrared spectra of different tourmaline species in the spectral range associated with the hydroxyl stretching modes, investigated through polarized Raman spectroscopy. Different lineshapes are observed for the OH spectra in uvite, fluor-uvite, magnesio-foitite, dravite and elbaite samples, and can be related to the coordination of OH in the two different structural V[O(3)]- and W[O(1)]-occupied sites. Local arrangements around the two different OH sites were assigned, and different ion substitutions for these five tourmaline species were identified. Our work with polarized Raman spectroscopy revealed that OH-stretching modes are described by totally symmetric, irreducible representations.

Large blue shift in the absorption spectra of BEH-PPV films containing gold nanoparticles

Abstract We report on a relatively large spectral blue shift observed in the absorption of conjugated polymer films of poly(2,5-bis(2′-ethyl-hexyl)-1,4-phenylenevinylene) containing gold colloidal nanoparticles (BEH-PPV/Au). The range of the photoluminescence (PL) emission energy, contrarily, does not change when compared to that of the pure BEH-PPV films. A large broadening effect in the PL peaks was observed for the BEH-PPV/Au film, which was attributed to a larger structural disorder induced in the polymer matrix by the colloidal nanoparticles. Micro-Raman spectroscopy revealed a broadening in the most intense phonon modes, which corroborates the disorder effects in the BEH-PPV/Au film structure. At low temperatures and also at higher excitation intensities, however, the PL vibronic peaks recover their relatively sharp characteristics, indicating a relative suppression of the disorder effects on the optical properties of the BEH-PPV/Au film. Our experimental results support the statement that gold colloidal nanoparticles, interspersed in the polymer matrix, can effectively lower the barrier for a crossover to a structurally disordered phase, resulting in a decrease in the effective conjugation length and consequently leading to a blue shift in the absorption.

Phase diagram of mixed LiK1-xRbxSO4 crystals

Neutron powder thermodiffractometry has been used to determine the phase diagram of LiK 1- x Rb x SO 4 mixed crystals ( x ≤ 0.50), between 50 and 800 K. We observed that, in this temperature range, these crystals undergo practically the same sequence of phase transitions as pure LiKSO 4 crystal. The main effects of the presence of rubidium ions in the potassium sites are seen in the variations of the critical temperatures and in the smearing of the first order transitions. We observed the narrowing of the region of the room temperature hexagonal phase with increasing Rb concentration. Its upper limit temperature decreases, while the lower one increases, favoring the trigonal phase. The low-temperature ferroelastic phase is absent for crystals with Rb concentration higher than c.a. 0.30.

Electrical conductivity of α-LiIO3 acid type crystals at 1 kHz

Abstract The temperature dependence of the electrical conductivity of α-LiIO3 acid type crystals is studied. By applying a very low amplitude electric field at 1 kHz and performing a continuous sampling of measurements, differences, reproducible for all the investigated samples, appeared between the first and subsequent heatings The anomalies occurring during the first heating are attributed mainly to inclusions of mother liquor, HIO3 and Li1-xHxIO3. The ‘intrinsic’ conductivity is measured after a first annealing at about 470 K; the activation energies are then calculated.

Poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) conjugated polymer domains in a thermoplastic polyurethane matrix

Self-sustained and spin-casting films formed from blends of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) conjugated polymer and thermoplastic poly-urethane (TPU) at different concentrations have been investigated. Scanning electron microscopy images of self-sustained films show the formation of circularlike domains of MEH-PPV in the TPU matrix, characterizing the blend material as heterogeneous. The circularlike domains in the spin-casting films were also observed by electric force microscopy (EFM) and by atomic force microscopy. The EFM, in particular, has allowed the differentiation of the MEH-PPV domains from the TPU matrix due to the electrical properties of the conjugated polymer. The MEH-PPV domains in the spin-casting films are much smaller in size than in the case of self-sustained films. The glass transition temperatures for the TPU and for the self-sustained blended films were obtained by temperature modulated differential scanning calorimetry. A very small variation in ...

Dielectric response of α-LiIO3 acid type crystals

Abstract α-LiIO 3 is often used as non-linear optical material. In view of its preparation, low pH aqueous solutions are used as mother solutions, which give crystals with some hydrogen incorporated to the lattice. Thus, dielectric characterization appears as a very important tool. In this work, the dielectric properties of acid type α-LiIO 3 crystals (conductivity, permittivity, loss-angle tangent) are investigated as functions of temperature and frequency, through an original method allowing seemingly continuous measurements and avoiding space charge contributions to the measurements. The results show the dipolar and low frequency conductivity contributions to the dielectric response. The role of the protons in this ionic conductor is also discussed.

Low temperature study of LiK1−xRbxSO4 mixed crystals

Abstract LiK1−xRbxSO4 solid solutions have been obtained for different concentrations x. Crystals with x = 0.10 and 0.50 have been investigated by Raman spectroscopy, in the temperature range 80 - 300K. The room temperature spectra show that the sample with x = 0.10 crystallizes in the hexagonal P63 LiKSO4 structure, while the crystal with x = 0.50 probably presents a trigonal P31c structure. The low temperature measurements show an increase of the hexagonal - trigonal critical temperature and a decrease of the ferroelastic transition temperature with increasing Rb content.

Electrical conductivity and micro-Raman scattering studies of ionic conduction in Li1−xHxIO3 solid solutions

Abstract Li 1− x H x IO 3 solid solutions have been investigated by ac electrical conductivity and micro-Raman techniques, for x ≤0.32. The presence of protons leads to a continuous reduction of the anisotropic intrinsic conduction of the system. The in-plane conduction mechanism would be by Li + vacancy hopping, while a mixed process by interstitial Li + and H + would account for the conduction along the c -axis. The solid solutions undergo the same phase transition sequence as the pure crystal, but the presence of the protons shifts the transition temperatures to lower values. Micro-Raman spectroscopy has been successfully introduced to demonstrate that proton mobility occurs preferentially along the c -axis.