Behavior of Supercritical Fluids across the "Frenkel Line".

The "Frenkel line" (FL), the thermodynamic locus where the time for a particle to move by its size equals the shortest transverse oscillation period, has been proposed as a boundary between recently discovered liquid-like and gas-like regions in supercritical fluids. We report a simulation study of isothermal supercritical neon in a range of densities intersecting the FL. Specifically, structural properties and single-particle and collective dynamics are scrutinized to unveil the onset of any anomalous behavior at the FL. We find that (i) the pair distribution function smoothly evolves across the FL displaying medium-range order, (ii) low-frequency transverse excitations are observed below the "Frenkel frequency", and (iii) the high-frequency shear modulus does not vanish even for low-density fluids, indicating that positive sound dispersion characterizing the liquid-like region of the supercritical state is unrelated to transverse dynamics. These facts critically undermine the definition of the FL and its significance for any relevant partition of the supercritical phase.

Local structure of solid Rb at megabar pressures.

We have investigated the local and electronic structure of solid rubidium by means of x-ray absorption spectroscopy up to 101.0 GPa, thus doubling the maximum investigated experimental pressure. This study confirms the predicted stability of phase VI and was completed by the combination of two pivotal instrumental solutions. On one side, we made use of nanocrystalline diamond anvils, which, contrary to the more commonly used single crystal diamond anvils, do not generate sharp Bragg peaks (glitches) at specific energies that spoil the weak fine structure oscillations in the x-ray absorption cross section. Second, we exploited the performance of a state-of-the-art x-ray focussing device yielding a beam spot size of 5 × 5 µm(2), spatially stable over the entire energy scan. An advanced data analysis protocol was implemented to extract the pressure dependence of the structural parameters in phase VI of solid Rb from 51.2 GPa up to the highest pressure. A continuous reduction of the nearest neighbour distances was observed, reaching about 6% over the probed pressure range. We also discuss a phenomenological model based on the Einstein approximation to describe the pressure behaviour of the mean-square relative displacement. Within this simplified scheme, we estimate the Grüneisen parameter for this high pressure Rb phase to be in the 1.3-1.5 interval.

Dynamics and Thermodynamics beyond the critical point.

Sudden changes in the dynamical properties of a supercritical fluid model have been found as a function of pressure and temperature (T/T(c) = 2-5 and P/P(c) = 10-10(3)), 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.

Lattice dynamics of dense lithium.

We report low-frequency high-resolution Raman spectroscopy and ab-initio calculations on dense lithium from 40 to 200 GPa at low temperatures. Our experimental results reveal rich first-order Raman activity in the metallic and semiconducting phases of lithium. The computed Raman frequencies are in excellent agreement with the measurements. Free energy calculations provide a quantitative description and physical explanation of the experimental phase diagram only when vibrational effect are correctly treated. The study underlines the importance of zero-point energy in determining the phase stability of compressed lithium.

High-pressure structural and vibrational study of PbZr0.40Ti0.60O3.

The high-pressure structure and dynamics of PbZr0.40Ti0.60O3 were investigated by means of neutron diffraction, X-ray diffraction, and resonance Raman spectroscopy. The complex (P4mm, Cm, Cc, F1, F1) phase transition sequence is characterized by these techniques. On the basis of the results of structure refinements, the high-pressure behavior of the spontaneous polarization, the (Zr,Ti)O6 rotation angles, and the polarization rotation angle are obtained. Moreover, resonance Raman spectra combined with previous Raman data in the literature provide evidence that the pressure-induced transition to the monoclinic Cm space group and the above transition sequence terminating in a paraelectric state are general features of Pb(Zr(1-x)Ti(x))O3 (0.48 < or = x < or = 1).

High pressure solid state chemistry of carbon dioxide.

A review of experimental and theoretical studies performed over the past three decades on high pressure chemistry of solid CO2, at 0-80 GPa and 40-3000 K, is presented. Emphasis is placed on the recently discovered non-molecular covalent crystalline phase V, and its glassy counterpart a-CO2, along with other molecular phases, whose interpretation is crucial for determining the reaction path to non-molecular CO2. The matter is still under debate, and many open issues are outlined, such as the true reaction mechanism for forming phase V. Finally, we propose arguments to stimulate possible future research in a more extended P-T range. This work is a tutorial review and should be of general interest both for solid state chemistry and condensed matter physics communities.

Liquidlike behavior of supercritical fluids.

The high frequency dynamics of fluid oxygen has been investigated by inelastic x-ray scattering, at high pressures and room temperature. In spite of the markedly supercritical conditions (T approximately 2Tc, P>10(2)Pc), the sound velocity exceeds the hydrodynamic value of about 20%, a feature which is the fingerprint of liquidlike dynamics. The comparison of the present results with literature data obtained in several fluids allow us to identify the extrapolation of the liquid-vapor-coexistence line in the (P/Pc, T/Tc) plane as the relevant edge between liquidlike and gaslike dynamics. More interestingly, this extrapolation is very close to the non-metal-metal transition in hot dense fluids, at pressure and temperature values as obtained by shock wave experiments. This result points to the existence of a connection between structural modifications and transport properties in dense fluids.

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.

Crystalline post-quartz phase in silica at high pressure.

alpha-quartz, which has been reported to undergo pressure-induced amorphization, was found to transform to a monoclinic, crystalline phase when compressed to 45 GPa at room temperature in a close to hydrostatic, helium pressure medium. The x-ray powder diffraction data obtained could be indexed based on a monoclinic cell, and the intensities are in agreement with a P2(1)/c model structure built up of 3x2 zigzag chains of SiO6 octahedra. This new polymorph of silica, which is metastable under ambient conditions, has been isolated for the first time and is one of several possible competing dense forms containing octahedrally coordinated silicon.