A. Paolone

POLARONIC OPTICAL ABSORPTION IN ELECTRON-DOPED AND HOLE-DOPED CUPRATES

Polaronic features similar to those previously observed in the photoinduced spectra of cuprates have been detected in the reflectivity spectra of chemically doped parent compounds of high-critical-temperature superconductors, both {ital n} type and {ital p} type. In Nd{sub 2}CuO{sub 4{minus}{ital y}} these features, whose intensities depend both on doping and temperature, include local vibrational modes in the far infrared and a broad band centered at {approximately} 1000 cm{sup {minus}1}. The latter band is produced by the overtones of two (or three) local modes and is well described in terms of a small-polaron model, with a binding energy of about 500 cm{sup {minus}1}. Most of the above infrared features are shown to survive in the metallic phase of Nd{sub 2{minus}{ital x}}Ce{sub {ital x}}CuO{sub 4{minus}{ital y}}, Bi{sub 2}Sr{sub 2}CuO{sub 6}, and YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}}, where they appear as extra-Drude peaks. The occurrence of polarons is attributed to local modes strongly coupled to carriers, as shown by a comparison with tunneling results. {copyright} {ital 1996 The American Physical Society.}

Infrared synchrotron radiation: from the production to the spectroscopic and microscopic applications

Abstract This study reviews the various mechanisms exploited to produce infrared synchrotron radiation (IRSR). It shows that at long wavelengths (long when compared to the critical wavelength of the bending magnet in an electron storage ring), the radiation emitted from a bending magnet edge can be brighter than standard synchrotron radiation. For this purpose, we will discuss the various IRSR sources, namely the bending magnets, the wigglers, the undulator and the bending magnet edges. We will then briefly review a high-resolution study of isolated molecules in the far infrared, the detection of a very narrow Drude term in a high- T c superconductor, a description of ultra-high pressure experiments, an investigation of water encapsulated in non-ionic reverse micelles, and finally, a brief review of spatially resolved studies.

Preparation and Characterization of Nanocomposite Polymer Membranes Containing Functionalized SnO2 Additives

In the research of new nanocomposite proton-conducting membranes, SnO2 ceramic powders with surface functionalization have been synthesized and adopted as additives in Nafion-based polymer systems. Different synthetic routes have been explored to obtain suitable, nanometer-sized sulphated tin oxide particles. Structural and morphological characteristics, as well as surface and bulk properties of the obtained oxide powders, have been determined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) and Raman spectroscopies, N2 adsorption, and thermal gravimetric analysis (TGA). In addition, dynamic mechanical analysis (DMA), atomic force microscopy (AFM), thermal investigations, water uptake (WU) measurements, and ionic exchange capacity (IEC) tests have been used as characterization tools for the nanocomposite membranes. The nature of the tin oxide precursor, as well as the synthesis procedure, were found to play an important role in determining the morphology and the particle size distribution of the ceramic powder, this affecting the effective functionalization of the oxides. The incorporation of such particles, having sulphate groups on their surface, altered some peculiar properties of the resulting composite membrane, such as water content, thermo-mechanical, and morphological characteristics.

The charge order transition and elastic/anelastic properties of LiMn2O4

We studied the charge ordering transition of LiMn2O4 by means of anelastic spectroscopy, measuring the Young’s modulus, E � ,a nd the elastic energy loss function, Q −1 .O ur measurements confirm the occurrence of a phase transition at 296 K on cooling, showing a large hysteresis on heating. The anelastic spectroscopy results also indicate that the transformation i sc har acterized by th ec oexistence of two phases. The shape of the real part of the Young’s modulus seems to indicate that the elastic constant which is more strongly affected by the transition is c44, like in the case of Fe3O4 .T hetime evolution

Stabilization of different conformers of bis(trifluoromethanesulfonyl)imide anion in ammonium-based ionic liquids at low temperatures.

The infrared absorption spectra of two ionic liquids with bis(trifluoromethanesulfonyl)imide (TFSI) as an anion and ammonium with different alkyl chains as cations are reported as a function of temperature. Using the comparison with ab initio calculations of the infrared-active intramolecular vibrations, the experimental lines were ascribed to the various ions composing the ionic liquids. In the liquid state of the samples, both conformers of the TFSI ion are present. In the solid state, however, the two conformers survive in N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI), while only cis-TFSI is retained in N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (TMHA-TFSI). We suggest that the longer alkyl chains of the former compound stabilize the less stable conformer of TFSI by means of stronger interactions between anions and cations.

Polaron imprints in the infrared spectra of Nd2CuO4−y

Abstract In addition to four Eu phonons, Nd2CuO4-y single crystals show far-infrared reflectivity peaks which depend on temperature and y. They provide evidence for the insurgence of local modes induced by doping. The so called d- or J-band at ∼ 1000 cm-1 can be partially resolved, and shown to be produced by combination bands of those local modes. These results provide direct evidence for polaron formation in this compound.

Phase Transitions of PYR14-TFSI as a Function of Pressure and Temperature: the Competition between Smaller Volume and Lower Energy Conformer

A detailed Raman study has been carried out on the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14-TFSI) over a wide pressure (0-8 GPa) and temperature (100-300 K) range. The explored thermodynamic region allowed us to study the evolution of the system across different solid and liquid phases. Calculated Raman spectra remarkably helped in the spectral data analysis. In particular, the pressure behavior of the most intense Raman peak and the shape analysis of the ruby fluorescence (used as a local pressure gauge) allowed us to identify a liquid-solid transition around 2.2 GPa at T = 300 K. The low-frequency Raman signal as well as the absence of remarkable spectral shape modifications on crossing the above threshold and the comparison with the spectra of the crystalline phase suggest a glassy nature of the high-pressure phase. A detailed analysis of the pressure dependence of the relative concentration of two conformers of TFSI allowed us to obtain an estimate of the volume variation between trans-TFSI and the smaller cis-TFSI, which is the favored configuration on applying the pressure. Finally, the combined use of both visual inspection and Raman spectroscopy confirmed the peculiar sequence of phase transitions observed as a function of temperature at ambient pressure and the different spectral/morphological characteristics of the two crystalline phases.

Charge-localization effects in the infrared transmittance of layered perovskites

In a previous study it has been shown that the formation of charged superlattices in Ni- and Mn-based perovskites opens a gap in their infrared polaronic background. This signature of charge localization is here used to compare the behavior of the polaronic carriers in Sr2MnO4, La2NiO4+y, Nd2CuO4−y and Nd1.85Ce0.15CuO4−y. It is shown that, as the temperature decreases, the carriers tend to localize in the nickelate and the manganite, to slightly increase their mobility in both cuprates.

Infrared response of ordered polarons in layered perovskites

We report on the infrared absorption spectra of three oxides, where charged superlattices have been recently observed in diffraction experiments. In La{sub 1.67}Sr{sub 0.33}NiO{sub 4}, polaron localization is found to suppress the low-energy conductivity through the opening of a gap and to split the {ital E}{sub 2{ital u}}-{ital A}{sub 2{ital u}} vibrational manifold of the oxygen octahedra. Similar effects are detected in Sr{sub 1.5}La{sub 0.5}MnO{sub 4} and in La{sub 2}NiO{sub 4+{ital y}}, with peculiar differences related to the type of charge ordering. {copyright} {ital 1996 The American Physical Society.}

The Complex Dance of the Two Conformers of Bis(trifluoromethanesulfonyl)imide as a Function of Pressure and Temperature

Absorbance spectra of two ionic liquids, the short alkyl chain N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI) and the longer chain N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (TMHA-TFSI) are reported as a function of pressure and temperature. The occurrence of various phase transitions is evidenced by the changes in the relative concentration of the cisoid and transoid conformers of their common TFSI anion. The infrared spectrum of TMPA-TFSI was measured at 300 K with an applied pressure varying over the 0-5 GPa range. Above 0.2 GPa only the trans conformer is detected, suggesting the occurrence of a pressure induced crystallization. When pressure is applied to TMHA-TFSI at T = 310 K, both TFSI conformers subsist up to ∼11 GPa. However, the clear change of their intensity ratio observed around 2 GPa, suggests the onset of a glass phase as supported by measurements carried out at 4.2 GPa along a cooling/heating cycle. A careful analysis of the spectra collected along different p-T thermodynamic paths shows the occurrence of a cold crystallization at 295 K on heating from 139 K along the p = 0.5 GPa isobar. The rich phase diagrams of the two ionic liquids is the result of the competition among the anion-cation intermolecular interactions, the lower energy of trans-TFSI with respect to cis-TFSI and the smaller volume of cis-TFSI with respect to trans-TFSI.