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PHYSICS AND CHEMISTRY AT HIGH PRESSURE--HIGH TEMPERATURE (Hi-PT)
 Stefano de Gironcoli Sandro Scandolo Giancarlo Trimarchi Nadia Bingelli Gernot Deinzer F. Mauri K. Uemoto R.M. WentzcovitchMain research lines:
Many important questions, in geophysics and planetary physics, as well as in applicative and basic science, concern the properties of matter at High-P and/or High-T conditions. These extreme conditions are however experimentally not easy to reach and control; and experimental probing and analysis face many difficulties due to sample size and accessibility. In this context atomistic simulations with well established predictive power (entirely ab-initio or performed using semi-empirical potential optimized on ab-initio data) have a very important role to play in assisting, complementing and guiding experiments. This is the main intellectual focus of the proposed research activity. The area of expertise of the scientists involved in the present activity covers a broad variety of materials and properties, studied using density Functional Theory (DFT) or DFT-derived approaches, and specifically a wide and well documented experience in the numerical study of the electronic, structural, elastic, and thermodynamical properties of bulk materials at ambient and high pressure. Many of us have been heavily involved in the development of new theoretical tools (such as e.g. density-functional perturbation theory for the study of lattice vibrations, or first principles variable cell-shape molecular dynamics for the study of pressure-induced phase transformations) and/or in the creation of sophisticated and flexible scientific software (such as the PWSCF-PHONON [1], FPMD [2] packages), mostly in the area of DFT pseudo-potential calculations. 1 THERMOELASTICITY AND PHASE DIAGRAMS OF MINERALS This research line builds on the previous experience of MRC-FORUM project and will address, by means of atomistic simulations, the thermophysics of candidate Earth's constituent phases, with an assessment of their structural and elastic properties and of their mutual phase equilibria at high pressure and temperature. We aim at providing reliable constraints and supplemental input to current Earth's composition models. Magnesium-wustite and iron-magnesium perovskite are the predominant phases in the Earth's lower mantle, accounting for more than 80% of Earth's volume and our study will initially address their properties. The thermodynamic relationship between these mineral phases will be studied by a combination of ab-initio electronic structure calculations and MonteCarlo simulations extending and improving the approach we introduced for the study of semiconductor alloys [3].In recent years we started a systematic study of elasticity of MgO and MgSiO3 [4] combining ab-initio lattice dynamical calculations [5] with the quasi-harmonic approximation for the free energy. This study will be completed and extended to other mineral phases. Diffusion and melting in ionic phases of geophysical interest will also be studied via ab-initio simulations [6] as well as with simulations using potentials tailored on ab-initio trajectories [7]. We will initially study pure mineral phases like silica, MgO, MgSiO3 and later on mixed systems as for instance SiO2 in presence of different cations. Finally, as many mineral phases are Mott insulating we would like to devise methods to integrate the DFT band structure description with more correlated approaches. 2 FLUIDS UNDER PLANETARY CONDITIONS Of great interest for planetary and also fundamental physics is the high-P and high-T behavior of simple molecular fluids and solids. The quest for hydrogen metallization, for example, has driven spectacular improvements in experimental and theoretical methodologies, which have in turn lead to the discovery of a rich phase diagram, and eventually of metallization, although so far only at high temperature. Beside their fundamental interest, the high-T and high-P structural, thermodynamical and electronic properties of simple molecular fluids are of enormous interest in planetary science [8], where they help constrain models of planetary evolution and structure, and interpret observational data from spacecrafts. Building on recent experience in the ab-initio simulation of some of these fluids [9] we plan to address in the future the properties of hot and compressed hydrogen, the major component of Jupiter and Saturn. Simulations will address the possibility of a molecular- to atomic-liquid transition under combined P and T, and will also investigate the miscibility of He in fluid hydrogen. In fact, although the overall He weight fraction in giant planets is believed to be about 25%, saturation and partitioning may alter locally this proportion, with important consequences for the structure and evolution of Jupiter and Saturn [10]. 3 HIGH PRESSURE MATERIALS PROCESSING AND SYNTHESIS High-P and high-T conditions are frequently used as a method to process and/or synthesize materials. One of the most brilliant examples is the well known shock-induced synthesis of diamond from graphite, first achieved in the 50s, and still the more profitable route to synthesize diamond industrially. More recently, the attention of high-P researchers has shifted from the efficient production of known materials, to the synthesis of completely new materials [11], possibly with tunable properties. The first example is the pressure-induced amorphization of quartz, a process that yields, when the product is recovered to ambient conditions, a denser form of glass, whose properties are only vaguely understood as of now [12]. Molecular dynamics simulations and lattice dynamics calculations [13] will be used in order to elucidate the mechanism of pressure-induced glass formation in silica and also in ice, where a similar phenomenology was observed for the first time. The second example consists in the attempt to design ways to stabilize the recently observed polymeric phase of nitrogen, produced under pressure, but not yet successfully quenched to ambient conditions [14]. Considerations borrowed from inorganic chemistry, as well as preliminary calculations, suggest that transition elements may help stabilize the polymeric over the molecular phase of nitrogen, which may open the route for practical applications of this new polymeric species. The fulfillment of this research project will require a substantial effort in terms of human and computational resources. Actually the whole Activity could not function without the technical support and the computational infrastructure by the IT-MC Activity. In addition, the Activity will benefit of specific expertises that can be found readily in other Activities of the Centre. For instance, the experience of the many-body group will be valuable in devising new approximations to accurately describe Mott insulating mineral phases, or the interaction with the large system group will be of help in developing efficient simulation tools to study alloys and/or amorphous phases. Likewise, methodological advances developed within the present Activity could be of interest for other ones. Similar techniques can be applied to high temperature studies of surfaces and minerals and the two activities will have mutual advantage from the interaction. As for the interaction with other institutions, we mention that an experimental facility for high-P physics is currently being set up at the ELETTRA synchrotron light source in Trieste, to be operational by end of 2002. We plan to establish a close interaction with this experimental group that will also help strengthen our interaction with the national geophysics community. Although very strong scientific communities exist in Italy both in the field of geophysics and condensed matter theory, little systematic attempt has been done until recently to bridge the two communities. We believe this is due to a large extent to the absence in Italy of an institutional framework in which such interaction could grow and develop. We expect that the existence of the Centre and the possibility to plan the activity on a 5-year basis will make this interaction really possible and we are confident that it will have an important and positive cultural impact on the two communities. BIBLIOGRAPHY [1] S. Baroni, A. Dal Corso, S. de Gironcoli, and P. Giannozzi, http://www.pwscf.org . [2] C. Cavazzoni and G. L. Chiarotti, A Parallel and Modular Car-Parrinello Code , Comp. Phys. Comm. 123, 56 (1999). [3] S. de Gironcoli, P. Giannozzi, and S. Baroni, Structure and Thermodynamics of SiGe Alloys from Ab-Initio Monte Carlo Simulations , Phys. Rev. Lett. 66, 2116 (1991). [4] B.B. Karki, et al., First-principles determination of elastic anisotropy and wave velocities of MgO at lower mantle conditions , Science 286, 1705 (1999); B.B. Karki, et al., First principles thermoelasticity of MgSiO3-perovskite: consequences for the inferred properties of the lower mantle , Geophys. Res. Lett. 89, 2699 (2001). [5] S. Baroni, P. Giannozzi, and A. Testa, Green's Function Approach to Linear Response in Solids , Phys. Rev. Lett. 58, 1861 (1987); S. Baroni, S. de Gironcoli, A. Dal Corso, and P. Giannozzi, Phonons and related properties of extended systems from density functional perturbation theory , Rev. Mod. Phys. 73, 515 (2001). [6] M. Bernasconi, et al., First-principle constant pressure molecular dynamics , J. Phys. Chem. Solids 56, 501 (1995). [7] A. Laio, et al., Physics of iron at Earth's core conditions, Science 287, 1027 (2000). [8] T. Guillot, Interiors of Giant Planets Inside and Outside the Solar System (Review) , Science 286, 72 (1999) [9] F. Ancilotto, et al., Dissociation of Methane into Hydrocarbons at Extreme (Planetary) Pressure and Temperature , Science 275, 1288 (1997); C. Cavazzoni, et al., Superionic and metallic states of water and ammonia at giant planet conditions , Science 283, 44 (1999). [10] D.J. Stevenson, States of matter in massive planets , J. Phys.: Cond. Matter 10, 11227 (1998) [11] S. Serra, et al., Pressure-induced solid carbonates from molecular CO2 by computer simulations , Science 284, 788 (1999); V. Iota, et al., Quartzlike Carbon Dioxide: An Optically Nonlinear Extended Solid at High Pressures and Temperatures , Science, 283, 1510 (1999). [12] R.J. Hemley, J. Badro, and D.M. Teter, in Physics Meets Mineralogy: Condensed-Matter Physics in Geosciences , eds. H. Aoki, Y. Syono and R. J. Hemley, (Cambridge University Press, New York, 2000). [13] S. Baroni and P. Giannozzi, High pressure lattice instabilities and structural phase transformations in solids from ab-initio lattice dynamics', in High pressure materials research , ed. R.M. Wentzcovitch et al., Mat.Res.Soc.Symp.Proc. 499, 233 (1998). [14] M.I. Eremets, et al., Semiconducting non-molecular nitrogen up to 240 GPa and its low-pressure stability , Nature 411, 170 (2001). |
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