标题： 从元动力学中柯石英致密化的动力学路径 显示英文标题

标题： Kinetic pathways of coesite densification from metadynamics

摘要： 我们研究了柯石英在35 GPa以上的压缩，远超过其向八面体相的平衡转变压力（8 GPa转变为斯石英）。 常温下的实验表明，在30 GPa以下，亚稳的柯石英结构仅发生轻微的位移变化（柯石英-II和柯石英-III），而硅原子保持四配位。 超过30 GPa后，开始发生重构转变，从柯石英的复杂结构沿着不同的路径进行。 除了非晶化外，还观察到了两种不同的晶体结果。 一种是缺陷高压八面体相的形成（Hu等，2015），另一种是柯石英-IV和柯石英-V的异常且复杂的致密相的形成，其中硅原子呈现四配位、五配位和六配位（Bykova等，2018）。 计算上捕捉这些结构转变是一个挑战。 在这里，我们展示了通过使用元动力学方法，结合基于机器学习的ACE势能函数（Erhard等，2024），以硅-氧配位数和体积作为通用集体变量，可以自然地观察到上述三种路径，从而得到实验中观察到的相。 我们描述了在转变路径中的原子机制。 虽然通向柯石英-IV的路径较为简单，但向八面体相的转变涉及两个步骤：首先，形成氧原子的六方密堆积子晶格，其中硅原子占据八面体位置，但八面体链并不形成规则的模式。 在第二步中，硅原子有序排列，链形成更规则的排列。 我们预测，在室温下通向柯石英-IV的路径更占优势，而在600 K时，八面体相的形成更有可能。 显示英文摘要

摘要： We study compression of coesite to pressures above 35 GPa, substantially beyond the equilibrium transition pressure to octahedral phases (8 GPa to stishovite). Experiments at room temperature showed that up to 30 GPa the metastable coesite structure develops only minor displacive changes (coesite-II and coesite-III) while the Si atoms remain 4-coordinated. Beyond 30 GPa, reconstructive transformations start, following different pathways from the complex structure of coesite. Besides amorphization, two different crystalline outcomes were observed. One is formation of defective high-pressure octahedral phases (Hu et al., 2015) and another one is formation of unusual and complex dense phases coesite-IV and coesite-V with Si atoms in 4-fold, 5-fold and 6-fold coordination (Bykova et al., 2018). Capturing these structural transformations computationally represents a challenge. Here we show that employing metadynamics with Si-O coordination number and volume as generic collective variables in combination with a machine-learning based ACE potential (Erhard et al., 2024), one naturally observes all three mentioned pathways, resulting in the phases observed experimentally. We describe the atomistic mechanisms along the transformation pathways. While the pathway to coesite-IV is simpler, the transformation to octahedral phases involves two steps: first, a hcp sublattice of O atoms is formed where Si atoms occupy octahedral positions but the octahedra chains do not form a regular pattern. In the second step, the Si atoms order and the chains develop a more regular arrangement. We predict that the pathway to coesite-IV is preferred at room temperature, while at 600 K the formation of octahedral phases is more likely.