2020 Fiscal Year Research-status Report
Constraining Earth's Lower Mantle Point Defect Chemistry from the Charge Disproportionation of Iron
| Project/Area Number |
20K14580
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| Research Institution | Ehime University |
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Principal Investigator |
RITTERBEX S 愛媛大学, 地球深部ダイナミクス研究センター, 特定研究員 (00791782)
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| Project Period (FY) |
2020-04-01 – 2024-03-31
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| Keywords | Ab initio simulations / Ferropericlase / Grain boundaries / Ideal shear strength / Grain boundary mobility / Plastic deformation / Earth's lower mantle / Super-Earth's mantle |
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| Outline of Annual Research Achievements |
The aim of this research project is to investigate the interactions between grain boundaries and point defects in lower mantle minerals. The properties of grain boundaries are critical to describe the mechanical behavior of polycrystalline mantle aggregates. Here, we start to focus on ferropericlase which is weaker compared to mantle silicates, affecting the bulk rheology of the lower mantle. We have carried out atomistic simulations based on the density functional theory to model the structures, energies and spin states of a series of [001] symmetrical tilt grain boundaries (STGB's) in ferropericlase as a function of pressure. Results have been used to determine the segregation energies of ferrous iron. We show that iron prefers specific STGB sites, altering the local high-to-low spin transition with respect to bulk ferropericlase. We investigated the mechanical behavior of this STGB by applying simple shear increments to the simulation cells in order to increase the stress on the inferface, needed to trigger the migration of the grain boundary. We found a strong pressure dependence on the mechanisms of grain boundary motion and the evolution of the ideal shear strengths. Between 30 and 400 GPa, we show that the grain boundary strength strongly varies both non-monotonously and monotonously, resulting in grain boundary hardening and weakening, respectively. The latter has likely implications for grain boundary-mediated plasticity in Earth's and super-Earth's mantles.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
The research progress develops rather smoothly. All planned first-principles simulations were conducted without problems on the FX1000 supercomputer at Nagoya University and the data were successfully analyzed at the Geodynamics Research Center of Ehime University. Due to the COVID-19 crisis, we were not able to undertake a planned visit to the University of Lille 1 to discuss collaborative research, causing a delay in the set-up of a joint paper. Instead, we started another international collaboration on the interaction between point defects and extended defects in diamond in line with this project's research objective on the Earth's mantle point defect chemistry. In summary, we were able to conduct the proposed research as initially planned and are preparing a manuscript discussing the obtained results.
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| Strategy for Future Research Activity |
The next target is to determine the partitioning of extrinsic point defects in ferropericlase between bulk and grain boundaries, relying on the quantification of the Gibbs free energies by using electronic structure simulations. In particular, we plan to focus on the role of different valence states of iron and its effect on grain boundary segregation and the influence on grain boundary strengths and mobilities. Grain boundaries form the main hosts of point defects and this step aims to understand the interaction between point defects and grain boundaries as a function of pressure in deep planetary interiors. These insights are expected to pave new ways towards a more comprehensive understanding of solid state flow and the chemical evolution of planetary mantles.
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| Causes of Carryover |
Due to the COVID-19 crisis, all planned business travels (i.e. conference travel; joint-research travel) were cancelled in FY2020. In addition, conference attendance costs were reduced as they were hold in a virtual format. This resulted in an incurring amount which will be used in the next fiscal year for additional external computational resources, broken hardware, extra RAM and supplementary SSD to accommodate atomistic simulations. It is also expected to be used for the purpose of publication fee.
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Research Products
(4 results)
