Materials Science
Synthesis of bulk hexagonal diamond
Researchers have synthesized the first bulk hexagonal diamond—a form of carbon long predicted to surpass cubic diamond in hardness—by compressing and heating graphite single crystals to produce millimeter-scale, highly ordered crystals.
You know how carbon can be arranged in different ways — like graphite in your pencil or diamonds in jewelry? Scientists have long suspected there's a third arrangement of carbon atoms, shaped like hexagons instead of cubes, that might be even harder than regular diamond. The problem was nobody could make a piece big enough to actually study. This team took ultra-pure graphite crystals, squeezed and heated them under very carefully controlled conditions, and finally grew chunks of this hexagonal diamond big enough to see, hold, and test. Think of it like finally baking a cake you've only ever seen in a recipe book for 60 years — and discovering it tastes almost exactly like the cake you already knew, but slightly better.
- 01
Successfully synthesized 100-micrometer to millimeter-sized bulk hexagonal diamond (HD) by compressing and heating high-quality graphite single crystals under controlled quasi-hydrostatic conditions
- 02
Demonstrated direct crystallographic transformation of graphite (1010) to HD (0002) and graphite (0002) to HD (1010), establishing an epitaxial relationship
- 03
Revealed that bulk HD consists of threefold intergrowth of 100-nm-sized crystals, predominantly HD with trace cubic diamond imperfections
- 04
Determined that interlayer bonds in HD are shortened relative to intralayer bonds, revealing the optimized structural configuration of HD
- 05
Measured that the hardness of bulk HD is only slightly higher than cubic diamond, providing the first reliable bulk mechanical property data for HD
Hexagonal diamond (HD), with anticipated physical properties superior than the known cubic diamond, has been pursued relentlessly since its inception 60 years ago. However, natural and synthetic HD has only been preserved as a highly disordered component in fragile, heterogeneous mixtures of other nanocarbon structures that precludes determination of bulk properties and identification of HD as a bona fide crystalline phase. Here we report the synthesis, recovery and extensive characterization of bulk HD by compressing and heating high-quality graphite single crystals under controlled quasi-hydrostatic conditions. We demonstrate the successful synthesis of 100-um-sized to mm-sized, highly ordered, bulk HD. We observed direct transformation of graphite (1010) orientation to HD (0002) and graphite (0002) to HD (1010). The bulk sample consists of threefold intergrowth of tightly knitted 100-nm-sized crystals, predominantly HD with trace imperfections of cubic diamond. The interlayer bonds in HD are shortened with respect to intralayer bonds to optimize the HD structure. Notably, the hardness of HD is only slightly higher than cubic diamond. We anticipate that purifying the precursor graphite carbon and fine-tuning the high pressure-temperature (P-T) synthesis conditions may lead to higher-quality HDs.