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.

中文翻译：

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压力对 MgO 中孪晶边界强度的不连续影响

氧化镁约占地球下地幔的 20%；因此，它的流变行为对地球的动力学和演化很重要。在这里，我们使用 DFT 计算和结构预测方法研究了 0 到 120 GPa 孪晶边界的强度。正如预期的那样，我们发现 310/[001] 界面剪切耦合迁移的能垒和临界应力随压力变化很大。然而，令人惊讶的是，孪晶边界也表现出突然的强强度不连续性，这可以随着压力的增加而削弱和加强边界。由于孪晶界迁移是 MgO 变形和地震衰减的建议机制，因此这些结果可能表明，由于晶界结构的转变，MgO 可以经历流变学的突然变化。