Materials Science
Bulk hexagonal diamond
Scientists synthesize bulk hexagonal diamond for the first time, revealing it is harder than conventional cubic diamond and resolving decades of controversy over its existence.
You probably know that diamonds are made of carbon atoms arranged in a specific pattern—like a perfectly stacked 3D grid. But imagine if those same carbon atoms could be stacked in a slightly different pattern, like a honeycomb, instead of a cube. Scientists have long believed this 'hexagonal diamond' exists because they found hints of it in rocks from meteorite impact sites, suggesting the extreme heat and pressure of a space rock smashing into Earth could create it. But nobody could make it in the lab or prove it was real on its own—until now. These researchers took a special form of super-flat graphite (the stuff in pencils), squeezed it really hard in just the right direction while heating it up, and successfully made millimeter-sized chunks of hexagonal diamond. They confirmed it's real, it's slightly harder than regular diamond, and it holds up to heat really well. Think of it as discovering a new flavor of the hardest material on Earth.
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Synthesis of millimetre-sized, phase-pure hexagonal diamond (HD) from highly oriented pyrolytic graphite (HOPG) compressed along the c-axis at elevated temperatures
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Advanced structural characterizations and theoretical simulations confirm the identity of HD and clarify the transformation pathway from graphite
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Bulk HD exhibits a slightly higher hardness than cubic diamond (CD) and high thermal stability
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The findings resolve the long-standing controversy on the existence of HD as a discrete carbon phase
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New insight provided into the graphite-to-diamond phase transition, opening avenues for practical use of HD in advanced technological applications
Known as the 'ultimate semiconductor', cubic diamond (CD) has gained substantial interest both scientifically and industrially. Its polymorph, hexagonal diamond (HD), is even more intriguing because of its fascinating properties associated with the meteorite impacts. As no solid experimental evidence has been provided to prove its existence, the physical properties of HD remain largely unexplored. Here we report the synthesis of millimetre-sized, phase-pure HD from highly oriented pyrolytic graphite (HOPG) compressed along the c-axis at elevated temperatures. Combining advanced structural characterizations and theoretical simulations, we confirm the identity of HD and clarify the transformation pathway from graphite. Bulk HD exhibits a slightly higher hardness than CD and high thermal stability. These findings resolve the long-standing controversy on the existence of HD as a discrete carbon phase and provide new insight into the graphite-to-diamond phase transition, paving the way for future research and practical use of HD in advanced technological applications.