ABSTRACT
Research Center SOFT-INFM-CNR, Universita` di Roma “La Sapienza”, I-00185, Roma, Italy and LENS, Via N. Carrara 1, I-50019 Sesto Fiorentino, Firenze, Italy
The knowledge of thermo-physical properties of materials at elevated pressure (P ) and temperature (T ) conditions is important in applied thermodynamics and geophysical/planetary science. Of particular interest is the study of fluid materials which, in the case of gaseous systems, are in the supercritical fluid state where not much is known about the evolution of physical properties. Properties such as elastic moduli, dispersion relations and damping of acoustic waves, viscosity and relaxation times are connected to the dynamics of density fluctuations [1], and can be determined, for example, by means of light scattering techniques [2, 3, 4, 5, 6, 7, 8]. The derived dynamical properties pertain to the hydrodynamic realm since the probe wavelength (a few hundred nanometer) is much larger than the coarse grained microscopic structure of any molecular system. Length scales comparable to the mean inter-particle distances can be assessed by coherent inelastic neutron (INS)[9, 10]and x-ray scattering (IXS)[11, 12, 13, 14]. The large size of neutron beams, however, limits such studies to relatively large samples, thus preventing the use of diamond anvil cells. These limitations can be overcome in the case of IXS, since undulator-based synchrotron X-rays can be focused down to small spot sizes, in the micrometer range. A few remarkable efforts have been performed in this direction using moderate pressures (<0.2 GPa) for liquid metals[15, 16]. Indeed, while studies on crystalline systems are routinely performed up to pressures of several tens of GPa [17], experiments on liquids are scarce [18, 19] and a quantitative visco-elastic analysis of the IXS spectra
has not been attempted, mainly due to problems of parasitic scattering from the sample environment and the diamonds, which is particularly critical in the study of light elements.
