Systematic theoretical study of [001] symmetric tilt grain boundaries in MgO from 0 to 120 GPa

Abstract

The properties of [001] symmetric tilt grain boundaries (STGBs) in magnesium oxide are investigated systematically with atomic-scale simulations. Their formation energies, atomic structures, and excess volumes are computed. STGBs are found to prefer a symmetric configuration, except for tilt angles larger than 67.4°, where the decomposition of the STGBs into ½[110] dislocations is preferred. Then, the effects of pressure are investigated from 30 up to 120 GPa, a pressure range relevant to the Earth's lower mantle. Pressure is found to change the atomic configuration of all investigated GBs, sometimes several times as pressure increases. We also find that these changes are irreversible, the GBs retaining their high-pressure configuration even after pressure is released. Implications for the deformation of ferropericlase in the conditions of the Earth's lower mantle are discussed.

The discontinuous effect of pressure on twin boundary strength in MgO

“…The interfacial energy of 1.47 J m −2 at 1.23 GPa found in this study is very similar to the previous ab initio study of 1.51 J m −2 by Verma and Karki (LDA) and somewhat below that of 1.95 J m −2 found in the embedded cluster study of McKenna and Shluger. There is also a small discrepancy between this work and the pair potential study of Hirel et al (2018), who found an interfacial energy of 1.63 J m −2 . Despite the LDA functional Harris et al 1996;Yokoi and Yoshiya 2018) and previously thought to be the most stable structure at intermediate pressures.…”

“…Structure (c) is more compact than the previous structure (a), with a small shift in the GB plane allowing Mg-O bonds to form whilst reducing the volume of the interface. Finally, structure (d) stable at pressure > 100 GPa is the most stable high-pressure structure and is observed in this and previous work (Verma and Karki 2010;Harris et al 1996;Hirel et al 2018). It features a relative shift in atomic planes parallel to the interfacial axis of rotation, resulting in a structure containing dislocation pipes.…”

“…consistently producing slightly higher interfacial energy about 0.3 J m −2 , the low-pressure results are generally in good agreement with each other, with all methods predicting the same low-pressure grain boundary, and with a zero pressure interfacial energy of about 1.5 J m −2 . At pressures greater than 5 GPa , previous studies (McKenna and Shluger 2009; Verma and Karki 2010; Harris et al 1996;Yokoi and Yoshiya 2018;Hirel et al 2018) suggest a transition to an atomically denser, symmetric structure, referred to in this study as structure (b) (see Fig. 2).…”

“…Unconstrained by system size, a recent empirical potentials study by Hirel and co-workers has expanded the range of studied interfacial misorientations to over 200. This extensive study not only matched geometric models for lowangle grain boundaries but also, established statistically significant trends on how the effects of pressure on interfacial energies and volumes vary as a function of misorientation (Hirel et al 2018). However, the dynamical behavioursstrength and mobility-of these interfaces have not been examined, despite their significance in the Earth's mantle.…”

Argon storage and diffusion in Earth’s upper mantle

“…However, as t′ and t″ are mostly controlled by temperature, the pressure effect should be very limited in the lithospheric mantle and the A-regime should always occur with depth increase. Regarding grain boundary complexion (e.g., Dillon & Harmer, 2008b), segregation between grain boundaries and the mineral lattice is pressure dependent (e.g., Hirel et al, 2018;Marquardt & Faul, 2018) since the grain boundary structure becomes less disordered, and denser with increasing confining pressure. Thus, grain boundary segregation should decrease with increasing pressure.…”