P. Postorino

Evidence of a pressure-induced metallization process in monoclinic VO2

Raman and combined infrared transmission and reflectivity measurements were carried out at room temperature (RT) on monoclinic VO2 over the 0-19 GPa and 0-14 GPa pressure ranges. Both lattice dynamics and optical gap show a remarkable stability up to P* approximately 10 GPa whereas subtle modifications of V ion arrangements within the monoclinic lattice, together with the onset of a metallization process via band gap filling, are observed for P >P*. Differently from P=0, where the VO2 metallic phase is found only in conjunction with the rutile structure above 340 K, a new RT metallic phase within a monoclinic structure appears accessible in the high pressure regime.

Neutron diffraction studies of H2O/D2O at supercritical temperatures. A direct determination of gHH(r), gOH(r), and gOO(r)

Neutron diffraction studies are reported on H2O at temperatures of 300 and 400 °C. The method of isotopic substitution is applied to three mixtures of H2O and D2O, and the diffraction data are used to determine the three radial distribution functions gHH(r), gOH(r), and gOO(r). These results can be used to discuss changes in nearest neighbor structure between water molecules, and to assess the degree of usefulness of representative (usually pairwise) model potentials.

Water above its boiling point: Study of the temperature and density dependence of the partial pair correlation functions. I. Neutron diffraction experiment

Neutron diffraction data on water, employing the technique of hydrogen/deuterium isotope substitution, are reported at three thermodynamic states above the boiling point. The structural information is analyzed in terms of the partial radial distribution functions, OO, OH, and HH, which are extracted from the neutron data. It is found that temperature affects mainly the medium and longer range order in the liquid, while density plays a significant role in controlling the degree of hydrogen bonding. To understand the structure of water obtained from these data it appears that many‐body cooperative interactions have to be correctly accounted for.

Unravelling the structure of protic ionic liquids with theoretical and experimental methods: ethyl-, propyl- and butylammonium nitrate explored by Raman spectroscopy and DFT calculations.

We present an analysis of gas-phase structures of small clusters of n-alkylammonium nitrates (ethyl, propyl, and butyl) together with vibrational Raman spectroscopy of their respective liquid phases. The assignment and interpretation of the resonant frequencies have been performed by comparison with high-quality ab initio (DFT) computations. The theoretical spectra are in excellent agreement with the measured ones and allow the interpretation and assignment of almost all the spectral features. A careful analysis of the vibrational frequencies and of the electronic structure of the compounds has provided additional information on various structural features and on the rather complex hydrogen bonding network that exists in such compounds. A geometric structure of the short-range local arrangement in the bulk phases is also proposed.

Hexagonal silicon realized

Silicon, arguably the most important technological semiconductor, is predicted to exhibit a range of new and interesting properties when grown in the hexagonal crystal structure. To obtain pure hexagonal silicon is a great challenge because it naturally crystallizes in the cubic structure. Here, we demonstrate the fabrication of pure and stable hexagonal silicon evidenced by structural characterization. In our approach, we transfer the hexagonal crystal structure from a template hexagonal gallium phosphide nanowire to an epitaxially grown silicon shell, such that hexagonal silicon is formed. The typical ABABAB... stacking of the hexagonal structure is shown by aberration-corrected imaging in transmission electron microscopy. In addition, X-ray diffraction measurements show the high crystalline purity of the material. We show that this material is stable up to 9 GPa pressure. With this development, we open the way for exploring its optical, electrical, superconducting, and mechanical properties.

Effect of the Al content on the optical phonon spectrum in Mg1-xAlxB2

Raman and infrared absorption spectra of Mg(1-x)Al(x)B(2) have been collected for 0<x<0.5 in the spectral range of optical phonons. The x-dependence of the peak frequency, the width and the intensity of the observed Raman lines has been carefully analized. A peculiar x-dependence of the optical modes is pointed out for two different Al doping ranges. In particular the onset of the high-doping structural phase previously observed in diffraction measurements is marked by the appearence of new spectral components at high frequencies. A connection between the whole of our results and the observed suppression of superconductivity in the high doping region is established.

Structure of the Molten Salt Methyl Ammonium Nitrate Explored by Experiments and Theory

We present an analysis of the structure of the monomethylammonium nitrate (MMAN) compound. Vibrational Raman spectroscopy and X-ray powder diffraction have been used to characterize the bulk phases of MMAN, and assignment of the resonant frequencies has been performed by ab initio (DFT) computations on small clusters of the compound. The theoretical spectra are in excellent agreement with the experimental ones and provide a means by which an interpretation of the hydrogen-bonding network that exists in such compound can be analyzed. In particular, we found that the spectrum of one of the solid phases is structurally very similar to that of the liquid. We present experimental evidence for the existence of such phase both from X-ray data and Raman spectra which, in turn, is easily interpreted with a one-to-one correspondence with the ab initio simulation of the small clusters. A geometric structure of the short-range local arrangement in these two bulk phases is therefore proposed.

Optical properties across the insulator to metal transitions in vanadium oxide compounds

We review the optical properties of three vanadium oxide compounds V(2)O(3), VO(2) and V(3)O(5), belonging to the so-called Magnéli phase. Their electrodynamics across a metal to insulator transition is investigated as a function of both temperature and pressure. We analyse thoroughly the optical results, with a special emphasis on the infrared spectral weight. This allows us to discuss the nature of the mechanisms driving the phase transitions in the three compounds, pointing out the role of electron-electron and electron-phonon interactions in the various cases.

Preparation and characterization of LaMnO3 thin films grown by pulsed laser deposition

We have grown LaMnO3 thin films on (001) LaAlO3 substrates by pulsed laser deposition. X-ray diffraction confirms that the films are only slightly relaxed and are oriented “square on square” relative to the substrate. The measured Raman spectra closely resemble that observed in bulk LaMnO3, which indicates no relevant distortions of the MnO6 octahedra induced by the epitaxial strain. Therefore, no detectable changes in the lattice dynamics occurred in our LaMnO3 strained films relative to the bulk case. Mn55 nuclear magnetic resonance identifies the presence of localized Mn4+ states. Superconducting quantum interference device magnetization measures TN=131(3)K and a saturation moment μ=1.09μB∕Mn, revealing a small concentration of Mn4+ and placing our films within the antiferromagnetic insulating phase.

Pressure tuning of electron-phonon coupling: The insulator to metal transition in manganites

An extended temperature and pressure-dependent investigation is carried out on a La0.75Ca0.25MnO3 sample exploiting the infrared absorption technique coupled to a diamond anvil cell. The pressure dependence of the insulator to metal transition temperature T(IM) is determined for the first time up to 11.2 GPa. The T(IM)(P) curve we propose to model the present data has an exponential-like behavior with an associated characteristic pressure P* playing the role of a decay constant. It is found that the equivalence between an external and an internal (chemical) pressure holds over a limited range of pressure, namely, P< or =2P*. Moreover, a certain universality character is associated with the proposed model curve in its ability to account for a large class of low-disorder manganites characterized by intermediate electron-phonon coupling.