Direct synthesis of millimeter-sized hexagonal diamond from graphite

Xiaohong Yuan; Guwen Chen; Yong Cheng; Shengcai Zhu; Fuyang Liu; Yujiao Ke; Kuo Hu; Yue Pan; Ming-Sheng Wang; Zhaodong Liu; Hu Tang; Bingbing Liu

doi:10.1016/j.scib.2025.03.003

Direct synthesis of millimeter-sized hexagonal diamond from graphite

Sci Bull (Beijing). 2025 Apr 30;70(8):1257-1263. doi: 10.1016/j.scib.2025.03.003. Epub 2025 Mar 3.

Authors

Xiaohong Yuan¹, Guwen Chen², Yong Cheng³, Shengcai Zhu⁴, Fuyang Liu⁵, Yujiao Ke⁶, Kuo Hu¹, Yue Pan¹, Ming-Sheng Wang³, Zhaodong Liu¹, Hu Tang⁷, Bingbing Liu⁸

Affiliations

¹ State Key Laboratory of High Pressure and Superhard Materials, Synergetic Extreme Condition High-Pressure Science Center, College of Physics, Jilin University, Changchun 130012, China.
² School of Materials, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China.
³ Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China.
⁴ School of Materials, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China. Electronic address: zhushc@mail.sysu.edu.cn.
⁵ Center for High Pressure Science and Technology Advanced Research, Beijing 100094, China.
⁶ School of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China.
⁷ State Key Laboratory of High Pressure and Superhard Materials, Synergetic Extreme Condition High-Pressure Science Center, College of Physics, Jilin University, Changchun 130012, China. Electronic address: hutang@jlu.edu.cn.
⁸ State Key Laboratory of High Pressure and Superhard Materials, Synergetic Extreme Condition High-Pressure Science Center, College of Physics, Jilin University, Changchun 130012, China. Electronic address: liubb@jlu.edu.cn.

PMID: 40082153
DOI: 10.1016/j.scib.2025.03.003

Abstract

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.

Keywords: Graphite; Hexagonal diamond; High pressure; MD simulation; Phase transformation.