[论文解读] Density functional modeling and total scattering analysis of the atomic structure of a quaternary CaO − MgO − Al 2 O 3 − SiO 2 (CMAS) glass: Uncovering the local environment of calcium and magnesium

Kai Gong, V. Ongun Özçelik+2 more|arXiv (Cornell University)|Jan 11, 2021

Glass properties and applications参考文献 101被引用 8

一句话总结

本研究结合密度泛函理论（DFT）与分子动力学模拟，对CaO–MgO–Al2O3–SiO2（CMAS）玻璃的原子结构进行建模，结果表明Ca作为电荷补偿剂，Mg作为网络修饰剂，且Mg存在团簇化现象并靠近非桥氧位点。该模型与实验总散射数据相符，并解释了反应性趋势，推动了对低CO2水泥及地质体系中CMAS玻璃行为的理解。

ABSTRACT

Quaternary $\mathrm{CaO} ext{\ensuremath{-}}\mathrm{MgO} ext{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3} ext{\ensuremath{-}}{\mathrm{SiO}}_{2}$ (CMAS) glasses are important constituents of the Earth's lower crust and mantle, and they also have important industrial applications such as in metallurgical processes, concrete production, and emerging low-${\mathrm{CO}}_{2}$ cement technologies. In particular, these applications rely heavily on the composition-structure-reactivity relationships for CMAS glasses, which are not yet well established. In this study, we combined force-field molecular dynamics (MD) simulations and density functional theory (DFT) calculations to generate detailed structural representations for a CMAS glass. The generated structures are not only thermodynamically favorable (according to DFT calculations) but also agree with experiments (including our x-ray and neutron total scattering data as well as literature data). Detailed analysis of the final structure (including partial pair distribution functions, coordination number, and oxygen environment) enabled existing discrepancies in the literature to be reconciled and has revealed important structural information on the CMAS glass, specifically (i) the unambiguous assignment of medium-range atomic ordering, (ii) the preferential role of Ca atoms as charge compensators and Mg atoms as network modifiers, (iii) the proximity of Mg atoms to free oxygen sites, and (iv) clustering of Mg atoms. Electronic property calculations suggest higher reactivity for Ca atoms as compared with Mg atoms, and that the reactivity of oxygen atoms varies considerably depending on their local bonding environment. Overall, this information may enhance our mechanistic understanding on CMAS glass dissolution behavior in the future, including dissolution-related mechanisms occurring during the formation of low-${\mathrm{CO}}_{2}$ cements.

研究动机与目标

• 利用先进计算与实验方法，解决四元CMAS玻璃局部结构长期存在的争议。
• 阐明Ca与Mg在CMAS玻璃网络修饰与电荷补偿中所起的独立作用。
• 建立与实验X射线和中子总散射数据一致的可靠CMAS玻璃原子尺度模型。
• 研究局部原子环境与反应性之间的关系，尤其与低CO2水泥形成相关。
• 为理解CMAS玻璃在地质与工业过程中的溶解与反应行为提供机制基础。

提出的方法

• 采用密度泛函理论（DFT）评估CMAS玻璃中候选原子结构的热力学稳定性。
• 利用力场分子动力学（MD）模拟生成CMAS玻璃的动态、平衡结构模型。
• 结合DFT与MD方法，对结构进行优化，直至与实验总散射数据（X射线与中子）匹配。
• 计算部分径向分布函数（PDF），以实验PDF验证模拟结构。
• 分析配位数、氧环境与局部键合，识别中程有序结构与团簇化现象。
• 计算电子性质，评估Ca与Mg阳离子及其周围氧原子的反应性差异。

实验结果

研究问题

• RQ1 Ca与Mg在CMAS玻璃中的精确局部原子环境是什么？它们在结构角色上如何不同？
• RQ2 模拟的结构特征与实验X射线和中子总散射数据相比如何？
• RQ3 Ca与Mg在铝硅酸盐玻璃网络中分别在电荷补偿与网络修饰中起什么作用？
• RQ4 CMAS玻璃中是否存在与Mg或Ca阳离子相关的中程有序特征或团簇化趋势？
• RQ5 氧原子的局部键合环境如何影响其反应性？Ca与Mg如何影响这一过程？

主要发现

• 经DFT与总散射数据验证的最终CMAS玻璃结构显示出明确的中程原子有序性，解决了文献中长期存在的模糊性。
• 钙（Ca）优先作为AlO4四面体的电荷补偿剂，通过电荷平衡稳定网络结构。
• 镁（Mg）主要作为网络修饰剂，强烈倾向于与非桥氧（NBO）位点结合。
• Mg阳离子表现出团簇化行为，表明网络结构中存在局部非均匀性。
• 电子结构计算显示Ca具有更高的本征反应性，这与其电子环境有关。
• 氧的反应性显著受局部配位环境影响：靠近Mg的非桥氧表现出独特的电子特性，影响溶解路径。