High-pressure dissociation of crystalline para-diiodobenzene: optical experiments and Car-Parrinello calculations

We have investigated the high-pressure properties of the molecular crystal para-diiodobenzene, by combining optical absorption, reflectance, and Raman experiments with Car-Parrinello simulations. The optical absorption edge exhibits a large red shift from 4 eV at ambient conditions to about 2 eV near 30 GPa. Reflectance measurements up to 80 GPa indicate a redistribution of oscillator strength toward the near-infrared. The calculations, which describe correctly the two known molecular crystal phases at ambient pressure, predict a nonmolecular metallic phase, stable at high pressure. This high-density phase is characterized by an extended three-dimensional network, in which chemically bound iodine atoms form layers connected by hydrocarbon bridges. Experimentally, Raman spectra of samples recovered after compression show vibrational modes of elemental solid iodine. This result points to a pressure-induced molecular dissociation process which leads to the formation of domains of iodine and disordered carbon.     

Electrochemical Approach for the Production of Layered Double Hydroxides with a Well-Defined Co/MeIII Ratio

Layered double hydroxides (LDHs) have been widely studied for their plethora of fascinating features and applications. The potentiostatic electrodeposition of LDHs has been extensively applied in the literature as a fast and direct method to substitute classical chemical routes. However, the electrochemical approach does not usually allow for a fine control of the M^II/M^III ratio in the synthesized material. By employing a recently proposed potentiodynamic method, LDH films of controlled composition are herein prepared with good reproducibility, using different ratios of the trivalent (Fe or Al) to bivalent (Co) cations in the electrolytic solution. All the obtained materials are shown to be effective oxygen evolution reaction (OER) catalysts, and are thoroughly characterized by a multi-technique approach, including FE-SEM, XRD, Raman, AES and a wide range of electrochemical procedures.     

Quantum critical scaling of the geometric tensors

Berry phases and the quantum-information theoretic notion of fidelity have been recently used to analyze quantum phase transitions from a geometrical perspective. In this Letter we unify these two approaches showing that the underlying mechanism is the critical singular behavior of a complex tensor over the Hamiltonian parameter space. This is achieved by performing a scaling analysis of this quantum geometric tensor in the vicinity of the critical points. In this way most of the previous results are understood on general grounds and new ones are found. We show that criticality is not a sufficient condition to ensure superextensive divergence of the geometric tensor, and state the conditions under which this is possible. The validity of this analysis is further checked by exact diagonalization of the spin-1/2 XXZ Heisenberg chain.     

Limit of hanger linearity in suspension footbridge dynamics: A new section model

The mechanical behaviour of suspension bridges is characterised by nonlinearities due to the main cables geometric effects and to the inability of the hangers to sustain compressive loads. The nonlinear effects due to hanger slackening are expected to increase in suspension footbridges due to lightweight decks, that is, low dead to live load ratio, and to shallow plate-girder decks with very low flexural and torsional stiffness. In this paper a new section model is proposed to study the limit of hanger linearity in lightweight suspension footbridges. The model is inspired to a four degrees-of-freedom model already proposed in the literature, but is expressed with a new formalism that allows some interesting properties to be outlined. Specifically, the expression of a particular frequency, herein called relative antiresonance frequency, as a function of the model generalised properties is derived: if the system is loaded with a harmonic force having that frequency, the linear behaviour of the hangers is assured for every value of the force amplitude. The proposed section model is applied to a footbridge benchmark subject to the pedestrian harmonic load and results are compared with those obtained through a nonlinear dynamic analysis on a 3D Finite Element model of the bridge.     

Computational study on the interaction of water with 2- and 3-hydroxypyridine and the corresponding pyridones

SCF –MO –LCAO ab-initio computations were carried out for the four tautomers 2-pyridone, 2-hydroxypyridine, 3-pyridone, and 3-hydroxypyridine interacting with a water molecule placed at different positions and orientations around each of them. The computed interaction energies were fitted by an analytical potential consisting of a 6–12 Lennard-Jones term plus a Coulombic one. The root-mean-square deviation of the fit is 2.5 kJ/mol. We report isoenergy contour maps for some important cross sections and obtain a qualitative trend of the solvation entropy, in agreement with available experimental data. The obtained potentials were used for Monte Carlo simulation of four clusters consisting of a tautomer molecule as solute and 50 water molecules, at 300 K. The problem of the transferability of fitting constants is also examined.     

T dependence of vibrational dynamics of water in ion-exchanged zeolites A: a detailed Fourier transform infrared attenuated total reflection study

In order to explore the influence of cation substitution on the vibrational dynamics of water molecules in zeolites, the evolution of structural properties of the O-H stretching band of water in fully hydrated Na-A and Mg-exchanged A zeolites has been studied, for different percentages of induced ion exchange, by Fourier transform infrared attenuated total reflection spectroscopy as a function of temperature. The differences revealed in the O-H stretching band shapes have been accounted by fitting the spectra as a sum of four components, corresponding to water molecules exhibiting different types of hydrogen bonding. The dependencies of the relative intensities, peak wave numbers, and bandwidths of the resolved components on temperature and Mg2+ content have been discussed. Evidence of the "structure-maker" role played by a zeolitic surface on physisorbed water, systematically enhanced by increasing the percentage of induced ion exchange, is given in the whole explored temperature range.     

Non-destructive identification of green and yellow pigments: the case of some Sicilian Renaissance glazed pottery

Selected decorated Renaissance ceramic fragments, found during the excavation of a Sicilian archaeological site (Caltagirone, Sicily, South Italy), have been studied by combining scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), and X-ray absorbance spectroscopy (XAS). The study was aimed at providing microchemical and microstructural characterization of the colored glazed coatings in order to elucidate the nature of the pigments in the decorative layers, and in the glaze itself. From the obtained results, the general perspective has been the identification of information to be used for a reliable recognition of the production techniques.