The discontinuous effect of pressure on twin boundary strength in MgO

Abstract

MgO makes up about 20% of the Earth's lower mantle; hence, its rheological behaviour is important for the dynamics and evolution of the Earth. Here, we investigate the strength of twin boundaries from 0 to 120 GPa using DFT calculations together with structure prediction methods. As expected, we find that the energy barrier and critical stress for shear-coupled migration of the 310/[001] interface vary strongly with pressure. However, what is surprising is that the twin boundary also exhibits sudden strong discontinuities in strength which can both weaken and strengthen the boundary with increasing pressure. Since twin boundary migration is a proposed mechanism for both deformation and seismic attenuation in MgO, these results may suggest that MgO can undergo sudden changes in rheology due to transitions in grain boundary structure. The multiplicity of interfaces, however, necessitates the need for further studies to examine the role that phase changes in grain boundary structure play in mediating polycrystalline plasticity in the Earth.

Atomic‐Scale Study of Intercrystalline (Mg,Fe)O in Planetary Mantles: Mechanics and Thermodynamics of Grain Boundaries Under Pressure

“…We obtain ∆ H GB = 3.47 J/m 2 at 30 GPa, consistent with ∆ H GB = 3.38 J/m 2 (LDA) at 31.8 GPa by van Driel et al. (2020), and slightly larger than ∆ H GB ∼ 2.9 J/m 2 found at 30 GPa based on classical MD with empirical model potentials (Hirel et al., 2019). This discrepancy can be likely attributed to the difference in methodology.…”

“…Under finite size and non‐equilibrium thermodynamic conditions, it might be possible that this crystal shear could act as an ideal nucleation site for dislocations (i.e., Frank‐Read sources). Both ground state structures and associated formation energies and excess volumes are found to be in good agreement with other studies up to a pressure of 120 GPa (van Driel et al., 2020; Verma & Karki, 2010), including the structural transition pressure determined at 90 GPa by van Driel et al. (2020) (Figure 2c).…”

“…This avoids the formation of Mg-Mg and O-O bonds and achieves optimal compaction of the void-like structure to minimize the work performed by pressure. This GB structure has been previously identified by the pioneering work of Verma and Karki (2010), but was first described in detail by van Driel et al (2020). We obtain ∆H GB = 3.47 J/m 2 at 30 GPa, consistent with ∆H GB = 3.38 J/m 2 (LDA) at 31.8 GPa by van Driel et al (2020), and slightly larger than ∆H GB ∼ 2.9 J/m 2 found at 30 GPa based on classical MD with empirical model potentials (Hirel et al, 2019).…”

“…This GB structure has been previously identified by the pioneering work of Verma and Karki (2010), but was first described in detail by van Driel et al (2020). We obtain ∆H GB = 3.47 J/m 2 at 30 GPa, consistent with ∆H GB = 3.38 J/m 2 (LDA) at 31.8 GPa by van Driel et al (2020), and slightly larger than ∆H GB ∼ 2.9 J/m 2 found at 30 GPa based on classical MD with empirical model potentials (Hirel et al, 2019). This discrepancy can be likely attributed to the difference in methodology.…”

“…On the other hand, quantum ab initio and atomistic classical molecular dynamics (MD) studies have progressively led to new insights into the effect of pressure on the microscopic structure, mobility and chemical segregation behavior of MgO GBs. It has been shown that pressure is able to induce structural GB transitions (Harris et al, 1996(Harris et al, , 1999(Harris et al, , 2001Hirel et al, 2019;Verma & Karki, 2010;Yokoi & Yoshiya, 2018), that GB strength can vary with pressure (van Driel et al, 2020) and that GBs can be preferential sources and sinks for ions and their defective counterparts compared to the bulk lattice (Karki et al, 2015;McKenna & Shluger, 2009).…”