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Study of the high-pressure helium phase diagram using molecular dynamics
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. (CMT)
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. (CMT)
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. (CMT)
2007 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 19, no 1, 016206- p.Article in journal (Refereed) Published
Abstract [en]

The rich occurrence of helium and hydrogen in space makes their properties highly interesting. By means of molecular dynamics ( MD), we have examined two interatomic potentials for He-4. Both potentials are demonstrated to reproduce high-pressure solid and liquid equation of state (EOS) data. The EOS, solid - solid transitions and melting at high pressures ( P) were studied using a two-phase method. The Buckingham potential shows a good agreement with theoretical and experimental EOS, but does not reproduce experimental melting data. The Aziz potential shows a perfect match with theoretical melting data. We conclude that there is a stable body-centred-cubic (bcc) phase for He-4 at temperatures ( T) above 340 K and pressures above 22 GPa for the Buckingham potential, whereas no bcc phase is found for the Aziz potential in the applied PT range.

Place, publisher, year, edition, pages
2007. Vol. 19, no 1, 016206- p.
National Category
Physical Sciences
URN: urn:nbn:se:uu:diva-97010DOI: 10.1088/0953-8984/19/1/016206ISI: 000243136300018OAI: oai:DiVA.org:uu-97010DiVA: diva2:171774
Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2012-03-05Bibliographically approved
In thesis
1. Studies of Material Properties using Ab Initio and Classical Molecular Dynamics
Open this publication in new window or tab >>Studies of Material Properties using Ab Initio and Classical Molecular Dynamics
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, material properties have been examined under extreme conditions in computer-based calculations.

The research on iron (Fe), nickel (Ni), and ferropericlase (Mg1-xFexO) are not only important for our understanding of the Earth, but also for an improved knowledge of these materials per se.

An embedded-atom model for Fe demonstrated to reproduce properties such as structure factors, densities and diffusion constants, and was employed to evaluate temperature gradients at Earth core conditions. A similar interaction together with a two-temperature method was applied for the analysis of shock-induced melting of Ni. For Mg1-xFexO, the magnetic transition pressure was shown to increase with iron content. Furthermore, the C44 softening with pressure and iron composition supports the experimentally observed phase transition for Mg0.8Fe0.2O at 35 GPa.

The properties of high density helium (He) is of great interest as the gas is one of the most abundant elements in the solar system. Furthermore, He and neon (Ne) are often used as pressure media in diamond anvil cells. The melting of He showed a possible fcc-bcc-liquid transition starting at T=340 K, P=22 GPa with a Buckingham potential, whereas the bcc phase was not seen with the Aziz form. For Ne, Monte Carlo calculations at ambient pressure showed very accurate results when extrapolating the melting temperatures to an infinite cluster limit. At high pressure, a one-phase ab initio melting curve showed a match with one-phase L-J potential results, which could imply a correspondence between ab initio/classical one-phase/two-phase calculations.

In the search for hard materials, ab initio calculations for four TiO2 phases were compared. Just as imposed by experiment, the cotunnite phase was found to be very hard. The anomalous elastic behavior of the superconducting group-V metals V, Nb, Ta was found to be related to shrinking nesting vectors and the electronic topological transition (ETT).

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. x, 85 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 422
Atomic and molecular physics, molecular dynamics, phase transitions, melting, elasticity, equation of state, metals, rare gases, Atom- och molekylfysik
urn:nbn:se:uu:diva-8626 (URN)978-91-554-7154-5 (ISBN)
Public defence
2008-04-25, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:15
Available from: 2008-04-04 Created: 2008-04-04Bibliographically approved

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Ahuja, Rajeev
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