1,720,966 research outputs found
Carbonia: the amorphous silicalike carbon dioxide.
No full textAmong the group IV elements, only carbon forms stable double bonds with oxygen at ambient conditions. At variance with silica and germania, the non-molecular single-bonded crystalline form of carbon dioxide, phase V, only exists at high pressure1, 2, 3, 4, 5, 6, 7, 8, 9. The amorphous forms of silica (a-SiO2) and germania (a-GeO2) are well known at ambient conditions; however, the amorphous, non-molecular form of CO2 has so far been described only as a result of first-principles simulations9. Here we report the synthesis of an amorphous, silica-like form of carbon dioxide, a-CO2, which we call 'a-carbonia'. The compression of the molecular phase III of CO2 between 40 and 48 GPa at room temperature initiated the transformation to the non-molecular amorphous phase. Infrared spectra measured at temperatures up to 680 K show the progressive formation of C–O single bonds and the simultaneous disappearance of all molecular signatures. Furthermore, state-of-the-art Raman and synchrotron X-ray diffraction measurements on temperature-quenched samples confirm the amorphous character of the material. Comparison with vibrational and diffraction data for a-SiO2 and a-GeO2, as well as with the structure factor calculated for the a-CO2 sample obtained by first-principles molecular dynamics9, shows that a-CO2 is structurally homologous to the other group IV dioxide glasses. We therefore conclude that the class of archetypal network-forming disordered systems, including a-SiO2, a-GeO2 and water, must be extended to include a-CO2
PRESSURE-INDUCED CHARGE DELOCALIZATION IN MANGANITES STUDIED BY MEANS OF INFRARED SPECTROSCOPY
No full textEVIDENCE OF PHASE SEPARATION IN THE PHASE DIAGRAM OF THE La0.75Ca0.25MnO3 MANGANITE BY INFRARED MEASUREMENTS.
No full textDynamics and Thermodynamics beyond the critical point
No full textSudden changes in the dynamical properties of a supercritical fluid model have been found as a function of pressure and temperature (T/Tc52-5 and P/Pc510-103), striving with the notion of a single phase beyond the critical point established by thermodynamics. The sound propagation in the Terahertz frequency region reveals a sharp dynamic crossover between the gas like and the liquid like regimes along several isotherms, which involves, at sufficiently low densities, the interplay between purely acoustic waves and heat waves. Such a crossover allows one to determine a dynamic line in the phase diagram which exhibits a very tight correlation with a number of thermodynamic observables, showing that the supercritical state is remarkably more complex than thought so far
The Widom line as the crossover between liquid-like and gas-like behaviour in supercritical fluids
No full textAccording to textbook definitions, there exists no physical observable able to distinguish a liquid from a gas beyond the critical point, and hence only a single fluid phase is defined. There are, however, some thermophysical quantities, having maxima that define a line emanating from the critical point, named the Widom line in the case of the constant-pressure specific heat. We determined the velocity of nanometric acoustic waves in supercritical fluid argon at high pressures by inelastic X-ray scattering and molecular dynamics simulations. Our study reveals a sharp transition on crossing the Widom line demonstrating how the supercritical region is actually divided into two regions that, although not connected by a first-order singularity, can be identified by different dynamical regimes: gas-like and liquid-like, reminiscent of the subcritical domains. These findings will pave the way to a deeper understanding of hot dense fluids, which are of paramount importance in fundamental and applied sciences. © 2010 Macmillan Publishers Limited. All rights reserved
Inelastic x-ray scattering from high pressure fluids in a diamond anvil cell
No full textWe present an experimental setup to study terahertz dynamics in fluids under high pressure, employing inelastic x-ray scattering and diamond anvil cell techniques. The use of a carefully designed vacuum chamber and the minimization and control of sources of parasitic scattering allowed circumventing previous limitations due to important empty cell contributions to the scattering signal. The successful implementation of our setup is demonstrated in the case of supercritical fluid argon, for which a full viscoelastic analysis yields the dispersion relation of sound waves, the generalized heat capacity ratio, and longitudinal viscosity. Our results are in excellent agreement with available experimental observables and molecular dynamics simulations
Pressure behavior of the sound velocity of liquid water at room temperature in the terahertz regime
No full textThe pressure evolution of the sound velocity in liquid water in the terahertz regime, c(infinity), between 0.05 and 0.88 GPa, at room temperature, has been investigated by synchrotron inelastic x-ray scattering in a diamond anvil cell. We confirm previous results showing that c(infinity) increases with density much less than the adiabatic sound velocity c(s), which is reasonably related to the known structural modifications in the hydrogen bond network. At variance with a previous study where an anomaly was found in the density evolution of c(infinity)-most likely due to the nonisothermal character of the study-the present work reveals a smooth behavior of c(infinity), which could provide a useful constraint to the current theories on liquid water
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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