Abstract
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 phase2,3,4. 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 (\(10\bar{1}0\)) orientation to HD (0002) and graphite (0002) to HD (\(10\bar{1}0\)). 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.
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Data availability
The datasets for this study are available in the source data of the corresponding figures. Requests for more materials should be addressed to H.-k. Mao. Source data are provided with this paper.
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Acknowledgements
We are thankful for the financial support from the National Natural Science Foundation of China under grant no. U1930401, the National Science Fund for Distinguished Young Scholars (grant no. T2225027), Shanghai Science and Technology Committee, China (no. 22JC1410300) and Shanghai Key Laboratory of Material Frontiers Research in Extreme Environments, China (no. 22dz2260800).
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L.Y., Z.Z., D.Z. and W.Y. carried out the synchrotron experiment. D.L. performed the TEM characterization. L.Y. and W.Y. performed the experimental data analysis. K.C.L. performed the Raman spectrum calculation. H.T., B.Y., G.N. and H.G. performed the multi-anvil synthesis and Vickers hardness measurements. Y.Y. prepared the TEM sample by FIB. L.Y. and K.C.L. carried out DFT simulations. L.Y., W.Y. and H.-k.M. wrote the manuscript. H.-k.M. conceived and designed the project. All authors contributed to the discussion of the results and revision of the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 XRD images of hexagonal graphite at different pressures before and after transition.
a, 13.2 GPa. b, 20.0 GPa. The incident X-ray beam is parallel to the c-axis of pristine graphite. The graphite (\(10\bar{1}0\)) and (\(11\bar{2}0\)) diffraction spots at 13.2 GPa and the corresponding spots appearing at 20.0 GPa are labelled by orange and green circles in a and b, respectively. c–j, Evolution of the (\(11\bar{2}0\)) and (\(10\bar{1}0\)) diffraction spots under pressures between 13.2 GPa and 20.0 GPa. A fuzzy peak epitaxy to the graphite (\(10\bar{1}0\)) peak with a smaller d-spacing appears and grows at the expense of graphite (\(10\bar{1}0\)) from 14.9 GPa, whereas a similar evolution can be seen from the graphite (\(11\bar{2}0\)) peak but with a larger d-spacing from 17.4 GPa. At 20.0 GPa, both graphite (\(10\bar{1}0\)) and (\(11\bar{2}0\)) peaks disappear and the new peaks with d-spacings of 2.061 Å and 1.223 Å form completely.
Extended Data Fig. 2 The integrated XRD patterns from the recovered HD sample with incident X-ray beam along (blue) and perpendicular to (black) the c-axis of starting graphite crystal.
The weak diffraction ring marked with * is from the glue for fixing the sample on the holder. In these two patterns, 2.175 Å and 2.080 Å diffraction peaks have the most intensity and separate well, which can be assigned to HD (\(01\bar{1}0\)) and (0002) diffraction peaks unequivocally.
Extended Data Fig. 3 TEM study of the microstructure in triple twinned HD domains.
a, SAD pattern using a large, selected-area aperture. b–d, Convergent electron diffraction patterns showing three sets of diffraction patterns from each set of triple-twinned domain. Each variant has a twofold [\(10\bar{1}0\)] diffraction pattern separated by 120°. e, Bright-field TEM image showing triple-twinned microstructure in our high P–T synthesized HD. f–h, Dark-field images using diffraction spots 1, 2 and 3 marked in a show that there exist three variants. HRTEM images across the interface of twinned domains. i, A large-area HRTEM image encompassing all three twin domains. The corresponding FFT pattern (l) exhibits pseudo-hexagonal symmetry with a lattice spacing of 2.08 Å, consistent with the XRD data shown in Fig. 1d. j, An HRTEM image of a single domain (area 1) along the [\(10\bar{1}0\)] zone axis. The corresponding FFT pattern (m) reveals orthorhombic symmetry. k, An HRTEM image of the overlapping region of three twinned domains (area 2), showing pseudo-hexagonal symmetry with a lattice spacing of 2.08 Å, as shown in the FFT pattern (n).
Extended Data Fig. 4 Tilt series of electron diffraction patterns from hexagonal and cubic diamond domains.
a, When sample is along the [\(10\bar{1}0\)] zone axis of HD, tilting the sample 30° along (0002) causes the diffraction spots to reach the [\(2\bar{1}\bar{1}0\)] zone axis, indicating in-plane sixfold symmetry. The [0001] zone axis diffraction was taken from a differently oriented grain. b,c, The simulated and experimental tilt series SAD patterns from hexagonal and cubic diamond samples. The precise matching in three-dimensional tilting SAD patterns confirm the crystallographic symmetry of HD. Tilt angles measured from experiments, as shown in the figure, match those from simulated diffraction patterns.
Extended Data Fig. 5 HRTEM and electron diffraction patterns of HD from three major zone axes.
a–i, HRTEM images (a–c), corresponding SAD patterns (d–f) and simulated electron diffraction patterns (g–i) from three main zone axes [0001], [\(2\bar{1}\bar{1}0\)] and [\(10\bar{1}0\)], respectively.
Extended Data Fig. 6 Statistical analysis of bond length and bond angle from the HRTEM along the [\(2\bar{1}\bar{1}0\)] zone axis.
A shorter (red) bond length 1.50 ± 0.08 Å along the (0001) direction and a longer (blue) bond length 1.58 ± 0.05 Å along the [0, 8, \(\bar{8}\), \(\bar{3}\)] direction was retrieved from 20 pairs and 18 pairs C–C distance, respectively, using the Laplacian of Gaussian blob detection algorithm implemented in the scikit-image package. A relatively larger bond angle distribution with an average of 112.1 ± 2.8° was obtained. To verify the accuracy of atomic position identification, the d-spacing values of the (\(01\bar{1}0\)) and (0001) planes (labelled with yellow dashed lines) were analysed to be 2.175 ± 0.063 Å and 2.080 ± 0.030 Å, averaged from the distances between 36 and 32 atom pairs, respectively, which match the d-spacing values from XRD measurement very well.
Extended Data Fig. 7 Rietveld refinement on the integrated XRD pattern from a recovered HD sample with lattice constants a = b = 2.5182 Å and c = 4.1780 Å.
The right panel presents the atom arrangement with two sets of bond length (1.5103 Å and 1.5649 Å) and two sets of bond angle (107.132° and 111.715°).
Extended Data Fig. 8 EELS spectra from HD and cubic diamond.
A very small pre-peak indicated by the yellow arrow in the HD EELS spectra can be seen. The estimated sp2/sp3 is around 3%, resulting from the amorphous surface layer owing to FIB sample preparation.
Extended Data Fig. 9 EELS spectra from the mixture of different polytypes of carbon.
a, EELS spectra summed from diamond and graphite with different percentages. b, EELS spectra summed from diamond and amorphous carbon with different percentages. All spectra are normalized with the maximum peak before the summation.
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Yang, L., Lau, K.C., Zeng, Z. et al. Synthesis of bulk hexagonal diamond. Nature 644, 370–375 (2025). https://doi.org/10.1038/s41586-025-09343-x
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DOI: https://doi.org/10.1038/s41586-025-09343-x
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