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Structural, vibrational and thermodynamic properties of MgSiO and MgSiO minerals from first-principles simulations

Hernández, ER; Brodholt, J; Alfè, D; (2015) Structural, vibrational and thermodynamic properties of MgSiO and MgSiO minerals from first-principles simulations. Physics of the Earth and Planetary Interiors , 240 (March ) 1 - 24. 10.1016/j.pepi.2014.10.007. Green open access

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Abstract

In this paper we report a computational study of the structural and vibrational properties of the Mg-end members forsterite, wadsleyite and ringwoodite of MgSiO, and akimotoite, majorite and the perovskite phase of MgSiO. Our calculations have been carried out in the framework of Density Functional Theory (DFT) using a plane wave basis set and the Projector-augmented Wave (PAW) method to account for the core electrons. All structures have been fully relaxed at a series of volumes corresponding to the pressure range relevant to the transition zone in the Earth's mantle, and at each volume the phonon frequencies have been obtained and classified. Using the quasi-harmonic approximation, we have estimated a series of thermodynamic properties for each structure, including the Gibbs free energy, from which we have computed approximate phase diagrams for MgSiO and MgSiO. In spite of our reliance on the quasi-harmonic approximation, which is expected to break down at high temperatures, our calculated phase diagrams qualitatively reproduce the main features expected from diagrams fitted to experimental data. For example, from the computed phase diagram for MgSiO we obtain a post-spinel boundary at P=22.1GPa at T=1873K, with a slope of -3.4MPa/K. This supports experimental results suggesting a relatively large slope rather than those favouring a much flatter one. It also suggests that vertical deflections of the 660km discontinuity due to thermal signatures from plumes and slabs should be similar to those at the 410km, and that a deflection of 35km as seen in recent seismic studies could be caused by a thermal anomaly as small as 330K. We also identify the triple point between the ringwoodite, ilmenite (plus periclase) and perovskite (plus periclase) phases to be at P=22.9GPa and T=1565K. Our results clearly illustrate the stringent requirements made on theoretical models in order to extract predictions compatible with the available experimental data.

Type: Article
Title: Structural, vibrational and thermodynamic properties of MgSiO and MgSiO minerals from first-principles simulations
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.pepi.2014.10.007
Publisher version: http://dx.doi.org/10.1016/j.pepi.2014.10.007
Language: English
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1460995
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