Defect segregation and migration at Mg2SiO4 forsterite grain boundaries: A first-principles study

Abstract:

Interfacial and/or surface defects are likely to be present in most naturally occuring as well as experimentally synthesized materials, including those in the Earth's interior. With regard to influence of these defects on material properties, they are suggested to enhance the diffusion and creep processes. The interfacial regions are also considered to serve as effective sinks for point defects including impurities. To assess some these properties, we have recently simulated (by employing density functional theory) native defects and impurities in Mg₂SiO₄ forsterite (which along with its high pressure-temperature phases constitute to be one of the most abundant materials of the Earth’s upper mantle) grain boundaries (GB). A number of energetically competitive tilt boundaries of types (0l1)/[100], (1l0)/[001] and (012)/[100] are considered for the study. In case of native defects, for all considered configurations, the results show a strong tendency to segregate to the interface region with an energy difference of (as high as) ~4 eV between the defect-in-bulk and defect-in-interface region. Similar overall trend is also observed for impurity cations (Ca and Al) and protons. Compression further facilitates segregation, supporting (our) previous results on MgO grain boundaries. In addition, finite temperature molecular dynamics data clearly shows enhanced dynamics for the impurity cations in the interface as compared to the host ions or impurity in bulk. In agreement with experimental suggestions, the present results demonstrate that GB regions can indeed serve as high-defect and high-mobility regions. The indication that the pressure enhances the stability of these interfacial defects may have profound implications for the Earth's mantle. Although the quantitative effect of temperature is not explored at this moment, it may further enhance these characteristics in the amorphous-like GB regions.