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According 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. [ABSTRACT FROM AUTHOR]

Recent infrared absorption measurements performed at different pressures and temperatures on the La0.75Ca0.25MnO3 manganite provided an estimate of the insulator-to-metal transition temperature T IM( P), i.e., the P- T phase diagram of a pseudocubic manganite. Here we consider in detail the P- and T-dependence of the absorption spectral weights n *. The n *( T, P) behavior gives evidence of phase-separation between conducting and insulating domains in the P-T phase diagram, and allows to establish the occurrence of a characteristic temperature T * ≅ 0.6 T IM. For T > T *, when insulating domains decrease on decreasing temperature or on increasing pressure, it is possible to distinguish a metallic region (connected metallic domains) for T * < T < T IM, and an insulating region (disconnected metallic domains) for T > T IM. Below T *, no reduction of the insulating domains, that may still exist, occurs. [ABSTRACT FROM AUTHOR]

The structure of the cristobalite-like polymorph of phosphorus oxynitride PON has been refined using neutron powder diffraction data. It is tetragonal, space group I&4 macr;2 d, Z=4. The four P–(O,N) distances are equal but the tetrahedron is compressed along c. In AX2 or ABX4 compounds, the tetragonal I&4 macr;2 d or I&4 macr; structure is obtained when the average ratio of the cation to anion radius is below 1.186, whereas the tetragonal P41212 or orthorhombic C2221 structure is obtained at low temperatures for larger ratios. The cell parameters of this PON polymorph have been determined as a function of hydrostatic pressure by in situ angle dispersive X-ray powder diffraction in a diamond-anvil cell. Under truly hydrostatic pressure, a strong anisotropic behavior is observed with the c parameter being nearly incompressible. Very slight anisotropic stress strongly modifies the high-pressure behavior. According to the pressure-temperature conditions of treatment, three phases, cristobalite-, moganite-, and quartz-like, have been obtained by quenching experiments, and the P– T phase diagram of PON was derived. The high-pressure behavior of the α-quartz, moganite, and cristobalite-like polymorphs of PON and SiO2 is discussed. [ABSTRACT FROM AUTHOR]

We report near-IR measurements of the electronic 3Σ− g→1 Δg(0,1) transitions of solid oxygen at high pressure and low temperature. Most of the P-T diagram was investigated up to the ε phase by means of isothermal and isobaric scans. The electronic spectra are a powerful tool to investigate the unique properties of solid oxygen, which is a localized-spin system. Magnetism of the high pressure δ phase plays a major role in determining the frequency of these bands, which are strongly temperature and density dependent. The analysis of this dependence allows us to model the spin correlation function and the intermolecular exchange integral. [ABSTRACT FROM AUTHOR]