Polycrystalline (Mg,Fe)O ferropericlase is the second most abundant mantle constituent of the Earth and possibly of super-Earth exoplanets. Its mechanical behavior is expected to accommodate substantial plastic deformation in Earth's lower mantle. While bulk properties of ferropericlase have been extensively studied, the thermodynamics of grain boundaries and their role on mechanical response remain largely unexplored. Here, we use density functional theory calculations to investigate mechanical behavior and thermodynamics of the {310}[001] grain boundary—a representative proxy for high-angle {hk0}[001] tilt grain boundaries—at relevant mantle pressures of the Earth and super-Earth exoplanets. Our results provide evidence that shear-coupled migration and grain boundary sliding are the dominant mechanisms of (Mg,Fe)O grain boundary mobility. We show that pressure-induced structural transformations of grain boundaries can trigger a change in the mechanism and direction of grain boundary motion. Significant mechanical weakening of the grain boundary is observed under multi-megabar pressures, caused by a change in the grain boundary transition state structure during motion. Our results identify grain boundary weakening in periclase as a potential mechanism for viscosity reductions in the mantle of super-Earths. We further demonstrate that structural grain boundary transitions control the spin crossover of Fe²⁺ in the grain boundaries. We model iron partitioning behavior between bulk and grain boundaries and predict equipartitioning to occur in μm size ferropericlase grains. Our findings suggest that the iron spin crossover pressure in ferropericlase may increase several tens of GPa by pressure-induced structural grain boundary transitions in dynamically active fine-grained lower mantle regions.

中文翻译：

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行星地幔中晶间 (Mg,Fe)O 的原子尺度研究：压力下晶界的力学和热力学

多晶 (Mg,Fe)O 铁方镁石是地球第二丰富的地幔成分，可能也是超级地球系外行星的地幔成分。其机械行为预计将适应地球下地幔的大量塑性变形。虽然方镁石的整体特性已被广泛研究，但晶界的热力学及其对机械响应的作用仍然很大程度上未被探索。在这里，我们使用密度泛函理论计算来研究{310}[001]晶界（大角度{ hk 0}[001]倾斜晶界的代表性代表）在地球相关地幔压力下的机械行为和热力学和超级地球系外行星。我们的结果证明剪切耦合迁移和晶界滑动是 (Mg,Fe)O 晶界迁移的主要机制。我们表明，压力引起的晶界结构转变可以引发晶界运动机制和方向的变化。在数兆巴压力下观察到晶界的显着机械弱化，这是由于运动过程中晶界过渡态结构的变化引起的。我们的研究结果表明，方镁石中的晶界弱化是超级地球地幔粘度降低的潜在机制。我们进一步证明结构晶界转变控制晶界中Fe ²⁺的自旋交叉。我们模拟了块体边界和晶界之间的铁分配行为，并预测微米尺寸的方镁石颗粒中会发生均分配。我们的研究结果表明，在动态活跃的细粒下地幔区域中，压力诱导的结构晶界转变可能会使铁方镁石中的铁自旋交叉压力增加数十 GPa。