Recently, the team of Professors Liu Bingbing and Yao Mingguang of Jilin University and Professor Zhu Shengcai of Sun Yat-sen University discovered a new path for graphite to form hexagonal diamond through the post-graphite phase under high temperature and pressure, and synthesized for the first time a high-quality, nearly pure hexagonal diamond bulk material, which is harder than cubic diamond and has good thermal stability. Relevant research results were published in the international academic journal "Nature·Materials".
In 1967, American scientists discovered a rare "super diamond" in a meteorite crater. It attracted much attention because it had a hexagonal crystal structure, was symbiotic in meteorites, and was harder. However, the artificial synthesis of pure-phase hexagonal diamond has been a scientific problem that has not been solved for a long time.
Previous research has proposed a new mechanism for the transformation of graphite to cubic diamond, and found that the formation of the sp3 carbon high-pressure phase structure is an important factor. The team was inspired that when exploring the artificial synthesis of hexagonal diamond, the high-pressure phase structure is likely to be the key.
To this end, the team designed a high-temperature and high-pressure experiment, using laser-heated diamond anvil technology to study the structural changes of graphite in situ under ultrahigh pressure and high temperature of 50 GPa. They found that graphite will form a post-graphite phase high-pressure structure in the high-pressure range, and then successfully obtained hexagonal diamond through local heating.
Research shows that synthetic hexagonal diamond has excellent physical properties, with a hardness 40% higher than that of natural diamond; its thermal stability can reach 1100°C in a vacuum environment, which is better than the 900°C of nanodiamond. The team further combined large-scale molecular dynamics theoretical simulations to reveal the key role of graphite layer stacking configuration in the formation of hexagonal diamond structure, confirming a new path for graphite to form hexagonal diamond through the post-graphite phase.
This achievement provides an effective way to artificially synthesize pure-phase hexagonal diamond, adding new members with better properties to superhard materials and new carbon materials. It is also of great significance for in-depth understanding of the specific sources of diamonds in meteorites and major geological events.