Synthesis of bulk hexagonal diamond

Hexagonal diamond (HD), with anticipated physical properties superior than the known cubic diamond, has been pursued relentlessly since its inception 60 years ago¹. 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^2-4. 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-µm-sized to mm-sized, highly ordered, bulk HD. We observed direct transformation of graphite ( $10\overline{1}0$ ) orientation to HD (0002) and graphite (0002) to HD ( $10\overline{1}0$ ). 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.