Yuan Xiaohong, Chen Guwen, Cheng Yong, Zhu Shengcai, Liu Fuyang, Ke Yujiao, Hu Kuo, Pan Yue, Wang Ming-Sheng, Liu Zhaodong, Tang Hu, Liu Bingbing
State Key Laboratory of High Pressure and Superhard Materials, Synergetic Extreme Condition High-Pressure Science Center, College of Physics, Jilin University, Changchun 130012, China.
School of Materials, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China.
Sci Bull (Beijing). 2025 Apr 30;70(8):1257-1263. doi: 10.1016/j.scib.2025.03.003. Epub 2025 Mar 3.
Diamond has the strongest three-dimensional network structure and its cubic configuration is extremely stable under high pressure, thus limiting the experimental synthesis of diamond polymorphs. Hexagonal diamond, a typical polymorph of diamond, has attracted considerable attention in recent decades, yet synthesizing pure and large-sized hexagonal diamond remains technically challenging, preventing an accurate understanding of its properties and formation mechanism. Here, we report the direct synthesis of millimeter-sized, nearly pure hexagonal diamond from graphite under high-pressure and high-temperature conditions using our developed high-pressure technique in a multi-anvil press. The synthesized hexagonal diamond is highly oriented polycrystalline, exhibiting an ultrahard hardness (165 ± 4 GPa) on (100) planes, which is ∼50% harder than single-crystal cubic diamond. Structural characterizations and molecular dynamics simulations indicate that hexagonal diamond is formed through a martensitic transformation process whereby hexagonal graphite is transformed into hexagonal diamond by sliding and then direct bonding between graphite sheets. Furthermore, we show that the transformations from graphite to cubic or hexagonal diamonds are strongly temperature-pressure dependent. With this understanding, we further synthesized cubic/hexagonal diamond composites with unusual heterostructures at a lower pressure. This work not only established a fundamental framework for high-pressure phase transformations in graphite but also provided insight into the structural evolution of two-dimensional materials at high pressures and a potent strategy for exploring their new high-pressure phases.
金刚石具有最强的三维网络结构，其立方构型在高压下极其稳定，因此限制了金刚石多晶型体的实验合成。六方金刚石是金刚石的一种典型多晶型体，近几十年来受到了广泛关注，但合成纯净且大尺寸的六方金刚石在技术上仍然具有挑战性，这阻碍了对其性质和形成机制的准确理解。在此，我们报告了使用我们在多砧压机中开发的高压技术，在高温高压条件下从石墨直接合成毫米级、近乎纯净的六方金刚石。合成的六方金刚石是高度取向的多晶体，在（100）面上表现出超硬硬度（165±4吉帕），比单晶立方金刚石硬约50%。结构表征和分子动力学模拟表明，六方金刚石是通过马氏体转变过程形成的，即六方石墨通过滑动然后石墨片层之间直接键合转变为六方金刚石。此外，我们表明从石墨到立方或六方金刚石的转变强烈依赖于温度和压力。基于这一认识，我们在较低压力下进一步合成了具有异常异质结构的立方/六方金刚石复合材料。这项工作不仅为石墨中的高压相变建立了基本框架，还深入了解了二维材料在高压下的结构演变，并为探索其新的高压相提供了有效策略。
