Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Non Equilibrium Dynamics and ComplexityMatter Under High PressureHigh pressure amorphyzation of carbon dioxideAmong the group IV elements, carbon is the uniquethat at ambient condition forms stable double bondswith oxygen. In contrast to the cases of SiO2 andGeO2 the nonmolecular tetrahedral crystalline formof CO2, phase V, only exists at high pressure.Similarly, while the amorphous phases of SiO2 (asilica)and GeO2 (a-germania) are well known andstable at room condition, the amorphous,nonmolecular, phase of CO2, although predicted byab-initio simulations, had not yet been discovered.Byusing a resistive heated Diamond Anvil Cell we couldsynthesize amorphous, silica-like, carbon dioxide.The non molecular amorphous phase of carbondioxide, a-CO2, that for similarity with othera-CO 2100 µmFig. 1. Photograph of Carbonia at 61 GPa. androom temperature. The sample is transparent andspatially homogeneous.amorphous oxide of the group IV we have called a-carbonia, was attained by compressing molecularphase III above 47 GPa at room temperature.In situ infrared spectra, measured with raisingtemperature up to 680 K, probe the progressiveformation of C-O single bonds and the simultaneousdisappearing of the molecular signatures. State-ofthe-artRaman and synchrotron x-ray diffractionmeasurements on the temperature quenched sampleshow the amorphous character of this material. Thecomparison with vibrational and diffraction patternsof amorphous silica and germania, shows that a-carbonia is homologous to those glasses. The staticstructure factor of a-CO2 has also been calculated byab initio techniques (Sandro Scandolo), reproducingthe main features of the experimental pattern.These findings do extend the scenario of archetypalnetwork-forming disordered systems such as a-silica,a-germania, a-Si and a-Ge, and water.References[1] M.Santoro, F.Gorelli, G.Ruocco, R.Bini,S.Scandolo and W.Crichton, “Carbonia: theamorphous silicalike carbon dioxide” (in preparation)AuthorsM.Santoro (a,b), F.A.Gorelli (a,b), , G.Ruocco (a,d),R. Bini (b,e), S.Scandolo (f) and W.Crichton (g).(a) INFM-CRS-Soft Matter (CNR), Univ. la Sapienza.(b) LENS, Univ. di Firenze. (c) Dip. di Fisica, Univ. diFirenze. (d) Dip. Fisica, Univ. La Sapienza, Roma. (e)Dip. Chimica, Univ. di Firenze. (f) ICTP-DNSC,Trieste, Italy (g) ESRF, Grenoble, FranceInelastic scattering of X rays from overcriticalfluids at high pressureThe possibility of performing inelastic X rayscattering measurements on low Z materials in theDiamond Anvil Cell has been recently demonstratedwith an experiment on liquid water (Krisch et alPhys. Rev. Lett. 89, 125502 (2002)). We performedan inelastic X ray scattering experiment on ID 28beamline of ESRF on overcritical fluid Oxygen at highpressure using a Diamond Anvil Cell. As the criticaltemperature of Oxygen is about 155 K and weperformed the measurements at room temperature,it implies a value of T/T c equal to about 2. Wecollected data at about 1, 3 and 5 GPa and comparedthe sound velocity with the adiabatic one measuredby Abramson et al (J. Chem. Phys. 110, 10493(1999)). It results a positive dispersion of about 20%at all pressures which has never been reported for anovercritical fluid, and on the contrary it is an ordinarybehaviour of liquids.This result has been compared to previous data onNitrogen, Neon and Mercury indicating that themetastable extension of the liquid-vapourcoexistence line beyond the critical point splits the PTdiagram in two regions corresponding to twodifferent regimes for what concerns the positivedispersion.This experiment has to be followed by similar oneson under/overcritical fluids at high pressure in orderto better elucidate this result.AuthorsF.A.Gorelli (a,b), M.Santoro (a,b), G.Ruocco (a,d),T.Scopigno (a,d) and A. Beraud (e).(a) INFM-CRS-Soft Matter (CNR), Univ. la Sapienza.(b) LENS, Univ. di Firenze. (c) Dip. di Fisica, Univ. diFirenze. (d) Dip. Fisica, Univ. La Sapienza, Roma. (e)ESRF (Grenoble), France (f) ICTP-DNSC, Trieste.Phase diagram and vibrational spectroscopy ofpolymers at high pressurePolyethylene is the most widely used polymericmaterial but very little is known about its propertiesat high pressure. We performed an infraredabsorption and Raman scattering study ofpolyethylene at high pressure and temperatures inthe range 0-60 GPa e 300-650 K. We also performedan X ray diffraction experiment in the same PT rangein order to have structural information to couple tovibrational data. The whole data sets are alsocompared with simulations provided by a theoreticalgroup in Trieste (Sandro Scandolo, ICTP)AuthorsL.Fontana (a), M.Santoro (a,b), F.A.Gorelli (a,b),S.Scandolo (c), R. Bini (a,d) and M.Hanfland (e).(a) LENS, Univ. di Firenze. (b) INFM-CRS-Soft Matter(CNR), Univ. la Sapienza. (c) ICTP-DNSC, Trieste.(d) Dip. Chimica, Univ. di Firenze. (e) ESRF(Grenoble), France.SOFT Scientific Report 2004-0680