The mechanism, synthesis and properties of hexagonal diamond

Minghao Wan; Shengcai Zhu

doi:10.1088/1674-4926/26010047

J. Semicond. > Just Accepted

RESEARCH HIGHLIGHTS

The mechanism, synthesis and properties of hexagonal diamond

Minghao Wan and Shengcai Zhu

+ Author Affiliations

Corresponding author: Shengcai Zhu, zhushc@mail.sysu.edu.cn

DOI: 10.1088/1674-4926/26010047CSTR: 32376.14.1674-4926.26010047

References

[1]	Bundy F P, Kasper J S. Hexagonal diamond: A new form of carbon. J Chem Phys, 1967, 46(9): 3437 doi: 10.1063/1.1841236
[2]	Németh P, Garvie L A J, Aoki T, et al. Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material. Nat Commun, 2014, 5: 5447 doi: 10.1038/ncomms6447
[3]	De A, Pryor C E. Electronic structure and optical properties of Si, Ge and diamond in the lonsdaleite phase. J Phys Condens Matter, 2014, 26(4): 045801 doi: 10.1088/0953-8984/26/4/045801
[4]	Salehpour M R, Satpathy S. Comparison of electron bands of hexagonal and cubic diamond. Phys Rev B, 1990, 41(5): 3048 doi: 10.1103/PhysRevB.41.3048
[5]	Zhu S C, Chen G W, Yuan X H, et al. Key for hexagonal diamond formation: Theoretical and experimental study. J Am Chem Soc, 2025, 147(2): 2158 doi: 10.1021/jacs.4c16312
[6]	Chen D S, Chen G W, Lv L, et al. General approach for synthesizing hexagonal diamond by heating post-graphite phases. Nat Mater, 2025, 24(4): 513 doi: 10.1038/s41563-025-02126-9
[7]	Yuan X H, Chen G W, Cheng Y, et al. Direct synthesis of millimeter-sized hexagonal diamond from graphite. Sci Bull, 2025, 70(8): 1257 doi: 10.1016/j.scib.2025.03.003
[8]	Thompson A P, Aktulga H M, Berger R, et al. LAMMPS - a flexible simulation tool for particle-based materials modeling at the atomic, meso, and continuum scales. Comput Phys Commun, 2022, 271: 108171 doi: 10.1016/j.cpc.2021.108171
[9]	Zhu S-C, Yan X-Z, Liu J, et al. A revisited mechanism of the graphite-to-diamond transition at high temperature. Matter, 2020, 3(3): 864 doi: 10.1016/j.matt.2020.05.013
[10]	Yang L X, Lau K C, Zeng Z D, et al. Synthesis of bulk hexagonal diamond. Nature, 2025, 644(8076): 370 doi: 10.1038/s41586-025-09343-x
[11]	Li J Y, Du G S, Zhao L L, et al. Experimental demonstration and transformation mechanism of quenchable two-dimensional diamond. Nat Commun, 2026, 17: 1244 doi: 10.1038/s41467-025-68005-8
[12]	Erskine D J, Nellis W J. Shock-induced martensitic phase transformation of oriented graphite to diamond. Nature, 1991, 349(6307): 317 doi: 10.1038/349317a0
[13]	Wheeler E J, Lewis D. The structure of a shock-quenched diamond. Mater Res Bull, 1975, 10(7): 687 doi: 10.1016/0025-5408(75)90052-5
[14]	Kurdyumov A V, Britun V F, Yarosh V V, et al. The influence of the shock compression conditions on the graphite transformations into lonsdaleite and diamond. J Superhard Mater, 2012, 34(1): 19 doi: 10.3103/S1063457612010029
[15]	Volz T J, Turneaure S J, Sharma S M, et al. Role of graphite crystal structure on the shock-induced formation of cubic and hexagonal diamond. Phys Rev B, 2020, 101(22): 224109 doi: 10.1103/PhysRevB.101.224109
[16]	Volz T J, Gupta Y M. Elastic moduli of hexagonal diamond and cubic diamond formed under shock compression. Phys Rev B, 2021, 103(10): L100101 doi: 10.1103/PhysRevB.103.L100101

Fig. 1. (Color online) Side-by-side microstructure and property evidence for bulk HD synthesized via two pathways. Route I (DAC, ~50 GPa, ~1800 K): (a)The nucleation and growth process of HD from AB-stacked graphite compressed to 40-40-50 GPa and laser heated to 1,800 K. The blue arrow shows the transformation process of post-graphite to HD during the heating process under high pressure. (b) Schematic diagram of the overall structure after phase transformation. (c-d) HRTEM images of two specimens along the c axis of pristine SG. (e) XRD patterns (axial vs radial) showing HD reflections without detectable CD peaks. (f) Vickers hardness measured along axial and radial directions^[6]. Route II (6-8-2, 20 GPa, 1773 K): (g−h) HRTEM images of the 20−1773 product along the [$01 \overline{1}0 $] and [$0001 $] directions of HD (insets: FFTs), highlighting lamellar HD microstructure with stacking-fault features and no discrete CD lattice observed in the examined regions. (i) Tauc plot giving an optical bandgap of HD compared with natural SCD. (j) TG/DSC curves in air indicating thermal stability up to 900 K^[7].

DownLoad: Full-Size Img PowerPoint

Fig. 2. (Color online) (a) Schematic of the epitaxial relationship between graphite and HD (b) HRTEM images along HD [$2 \overline{1}\overline{1}0 $] zone axes showing a large area of HD with AB stacking^[10]. (c−e) Atomistic observations of the 2D diamond quenched from HPHT conditions (c) Atomic arrangement of RG/CD hybrid interface, RG represents rhombohedral graphite. (d) Details of ABC stacking orders selected from the marked orange box in (c). The orange dashed lines highlight the RG/CD boundaries. (e) Atomic arrangement of HG/HD hybrid interfaces in 2D diamond. HG represents hexagonal graphite. The interfacial boundaries are highlighted by blue dashed lines. (f) EELS of the samples recovered from HPHT. A progressive increase in sp³ fraction from 71.3% to 89.9% correlates with enhanced σ^* peak intensity, as shown by the gradient blue shaded region. (g) Absorption spectra of ultrathin diamond with different sp³ percentages. (h) Raman spectra of ultrathin diamond sample taken after different annealing temperatures. The two dashed lines correspond to the characteristic Raman peaks of diamond (T_2g) and graphite (G), respectively^[11].

DownLoad: Full-Size Img PowerPoint

Table 1. Comparison of CD and HD

Material	Cubic diamond (CD)	Hexagonal diamond (HD)
Polytype/Stacking	ABCABC	AB′AB′
Space group	Fd-3m (No. 227)	P6₃/mmc (No. 194)
Primitive cell	2 atoms	4 atoms
Lattice constant	a= 3.56 Å	a = 2.52 Å, c = 4.12 Å
Bonding	sp³ tetrahedral C−C	sp³ tetrahedral C−C (same nearest-neighbor bonding; stacking differs)
Bandgap type	Indirect	Indirect
Bandgap magnitude	~5.4−5.5 eV	~4.5−4.8 eV
Optical response	Nearly isotropic	Strong polarization-dependent optical anisotropy
Static dielectric constant (diamond)	ε₀=5.7	ε_0,∥=6.319, ε_0,⊥=5.799

DownLoad: CSV

Table 2. Summary of representative experimental conditions in Refs. [5−7]

Ref.	Route	Starting Material	Pressure-Temperature Protocol	Stress/Pathway control
Ref. [5]	Kawai multi-anvil press (MAP)	HOPG	MAP at 20 GPa; heating ramp 50 K/min; hold 20 min; cool 100 K/min; decompression 1 GPa/h	quasi-uniaxial loading along graphite [0001]_g via precompressed Al₂O₃ tube
Ref. [6]	Diamond-Anvil-Cell (DAC) laser heating of compressed graphite	Single Crystal Graphite (SG)	SG compressed to50 GPa at room temperature; post-graphite forms above 26 GPa; laser heating up to 1800 K at 50GPa;	HD growth favored by post-graphite intermediate and temperature gradients;
Ref. [7]	6-8-2 multi-anvil assembly	HOPG	20 GPa; HD transformation completes near 1773 K; higher T leads to CD peaks	6-8-2 assembly integrates piston-cylinder (dense Al₂O₃) to compress mainly along z-axis
Phase identification relied on XRD/Raman/TEM (and PED/EELS where applicable) as reported in Refs. [5−7]. HD Lattice Parameters: a=2.511 Å and c=4.129 Å (Ref. [7], XRD Le Bail refinement)

DownLoad: CSV

[1]	Bundy F P, Kasper J S. Hexagonal diamond: A new form of carbon. J Chem Phys, 1967, 46(9): 3437 doi: 10.1063/1.1841236
[2]	Németh P, Garvie L A J, Aoki T, et al. Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material. Nat Commun, 2014, 5: 5447 doi: 10.1038/ncomms6447
[3]	De A, Pryor C E. Electronic structure and optical properties of Si, Ge and diamond in the lonsdaleite phase. J Phys Condens Matter, 2014, 26(4): 045801 doi: 10.1088/0953-8984/26/4/045801
[4]	Salehpour M R, Satpathy S. Comparison of electron bands of hexagonal and cubic diamond. Phys Rev B, 1990, 41(5): 3048 doi: 10.1103/PhysRevB.41.3048
[5]	Zhu S C, Chen G W, Yuan X H, et al. Key for hexagonal diamond formation: Theoretical and experimental study. J Am Chem Soc, 2025, 147(2): 2158 doi: 10.1021/jacs.4c16312
[6]	Chen D S, Chen G W, Lv L, et al. General approach for synthesizing hexagonal diamond by heating post-graphite phases. Nat Mater, 2025, 24(4): 513 doi: 10.1038/s41563-025-02126-9
[7]	Yuan X H, Chen G W, Cheng Y, et al. Direct synthesis of millimeter-sized hexagonal diamond from graphite. Sci Bull, 2025, 70(8): 1257 doi: 10.1016/j.scib.2025.03.003
[8]	Thompson A P, Aktulga H M, Berger R, et al. LAMMPS - a flexible simulation tool for particle-based materials modeling at the atomic, meso, and continuum scales. Comput Phys Commun, 2022, 271: 108171 doi: 10.1016/j.cpc.2021.108171
[9]	Zhu S-C, Yan X-Z, Liu J, et al. A revisited mechanism of the graphite-to-diamond transition at high temperature. Matter, 2020, 3(3): 864 doi: 10.1016/j.matt.2020.05.013
[10]	Yang L X, Lau K C, Zeng Z D, et al. Synthesis of bulk hexagonal diamond. Nature, 2025, 644(8076): 370 doi: 10.1038/s41586-025-09343-x
[11]	Li J Y, Du G S, Zhao L L, et al. Experimental demonstration and transformation mechanism of quenchable two-dimensional diamond. Nat Commun, 2026, 17: 1244 doi: 10.1038/s41467-025-68005-8
[12]	Erskine D J, Nellis W J. Shock-induced martensitic phase transformation of oriented graphite to diamond. Nature, 1991, 349(6307): 317 doi: 10.1038/349317a0
[13]	Wheeler E J, Lewis D. The structure of a shock-quenched diamond. Mater Res Bull, 1975, 10(7): 687 doi: 10.1016/0025-5408(75)90052-5
[14]	Kurdyumov A V, Britun V F, Yarosh V V, et al. The influence of the shock compression conditions on the graphite transformations into lonsdaleite and diamond. J Superhard Mater, 2012, 34(1): 19 doi: 10.3103/S1063457612010029
[15]	Volz T J, Turneaure S J, Sharma S M, et al. Role of graphite crystal structure on the shock-induced formation of cubic and hexagonal diamond. Phys Rev B, 2020, 101(22): 224109 doi: 10.1103/PhysRevB.101.224109
[16]	Volz T J, Gupta Y M. Elastic moduli of hexagonal diamond and cubic diamond formed under shock compression. Phys Rev B, 2021, 103(10): L100101 doi: 10.1103/PhysRevB.103.L100101