石墨碳高压相变的第一性原理动力学模拟计算研究

Phase transformation of graphite is among the most intriguing topics in materials science. Beside the high-pressure and high-temperature phase transformation to form diamond, the crystal structures of graphite compressed at room temperature and the corresponding structural transformation mechanism have been longstanding. The phase transformation process is highly complicated and remains poorly understood despite extensive past effort. Compressed graphite acts as a key gateway toward various diamond phases; an accurate mapping of its structural landscape is essential to understanding the intricate phase relations among various carbon allotropes. Experimentally, an unquenchable transparent and hard phase that has long been observed during the cold-compression (at room temperature) stage of the synthesis. Recent studies identified several structural forms for cold-compressed graphite, including a monoclinic M-carbon and an orthorhombic W-carbon, which provide a critical link for the graphite-to-diamond transformation. However, graphite's layered structure is conducive to the sliding and buckling of the graphitic sheets during the initial stages of compression, which introduces a high degree of complexity in the structural landscape of compressed graphite.. In present work, we perform by ab initio calculations a detailed study on the phase transformation of graphite-to-diamond under high-pressure: (1) we study the crystal structure and pathway of cold compressed graphite (such as Z-carbon and J-carbon) via the slip and buckling of carbon sheets; (2) we study the crystal structure and possible pathway to form new cubic carbon phase in graphite-originated phase transformation with a wide lattice range of 3.56-5.54 ? ; (3) we study the nanoscale conversion mechanism from the metastable compressed graphite phase to diamond structures. The present study should offer insights for understanding the complex structural landscape of compressed graphite and the versatile nature of carbon in forming a rich variety of structures under pressure.

碳元素是自然界中分布最为广泛的元素之一。单质碳通常以石墨和金刚石两种晶型存在。碳晶体结构及其相变的研究是理解其物性和应用的基础。但是长期以来关于层状石墨碳的高温高压相变机制一直困扰着实验和理论科学工作者。最近通过对石墨冷压相变的研究使得这一工作取得显著进展。我们通过第一性原理模拟计算研究了石墨碳的冷压相变机制并发现了两个新的三维正交碳亚稳相W-carbon和O-carbon。为了全面地理解从层状石墨到金刚石的高压相变机理，本研究拟通过第一性原理动力学模拟计算，从热力学和动力学两方面系统地研究石墨碳的高压相变机制及其形成的三维碳晶体结构的结构稳定性及相关物性。重点探讨（1）冷压石墨高压亚稳相的晶体结构及其相变过程；（2）高压下由石墨形成立方晶碳结构的动力学机制；（3）高温高压下从石墨亚稳相到金刚石的相变机制。

碳元素是自然界中分布最为广泛的元素之一。其特有的sp, sp2和sp3化学键赋予了其结构的多样性。但是长期以来关于层状石墨碳的高压相变机制一直困扰着实验和理论科学工作者。最近通过对石墨冷压相变的研究使得这一工作取得显著进展。通过第一性原理计算，我们系统地研究了石墨的高压相变机制以及所形成的三维碳晶体结构的结构稳定性和物性。提出了一个新的立方all-sp3 BC12碳和四个all-sp3超密碳结构t32, t32*, m32, m32*碳；提出了三个由螺旋碳链通过乙烯型碳碳双键(>C=C<)结合而成的all-sp2三维手性碳烯晶体结构cR6，cT8，Rh6碳；同时发现了一个新的node-line半金属all-sp2三维碳烯晶体结构BCO-C16碳；另外还发现一个具有sp2+sp3杂化键的H18金属碳和立方all-sp3 K6金属碳。系统地研究了从一维碳纳米管到三维类金刚石结构的高压重构机制；通过分子动力学模拟计算提出了一个新的石墨-金刚石相变机制“wave like buckling and slipping mechanism”（波状弯曲滑移机制）。在该项目的支持下，共发表论文19篇，其中PRL 2篇，PRB 3篇，JCP 3篇，Sci. Rep. 6篇。这些研究增进了我们对碳及有关实验现象和物性的认识和理解。