Coherent interfaces govern direct transformation from graphite to diamond

Abstract

Understanding the direct transformation from graphite to diamond has been a long-standing challenge with great scientific and practical importance. Previously proposed transformation mechanisms^1-3, based on traditional experimental observations that lacked atomistic resolution, cannot account for the complex nanostructures occurring at graphite-diamond interfaces during the transformation^4,5. Here we report the identification of coherent graphite-diamond interfaces, which consist of four basic structural motifs, in partially transformed graphite samples recovered from static compression, using high-angle annular dark-field scanning transmission electron microscopy. These observations provide insight into possible pathways of the transformation. Theoretical calculations confirm that transformation through these coherent interfaces is energetically favoured compared with those through other paths previously proposed^1-3. The graphite-to-diamond transformation is governed by the formation of nanoscale coherent interfaces (diamond nucleation), which, under static compression, advance to consume the remaining graphite (diamond growth). These results may also shed light on transformation mechanisms of other carbon materials and boron nitride under different synthetic conditions.

Fig. 1. XRD patterns and phase evolution diagram of graphite under HPHT.

a, XRD of samples recovered from 15 GPa and 1,200 °C, 1,400 °C, 1,600 °C and 2,000 °C. The pristine graphite is included for comparison. The coloured tags at the bottom indicate standard diffraction lines of graphite (HG) and cubic diamond (CD). b, Kinetic phase diagram of graphite under HPHT determined from the XRD results. Hexagons, pentagons and diamond symbols represent samples that are pure graphite, mixed phases containing CD and other metastable carbon phases, and pure diamond, respectively. Collectively, these data points define three regions as delineated by the solid lines. The dashed line is the established phase boundary between graphite and diamond.

Fig. 2. Microstructures of a sample recovered from 15 GPa and 1,200 °C.

a, Low-magnification BF-STEM image showing nanoscaled diamond (D) domains embedded in graphite (G). b, High-resolution HAADF-STEM image of graphite domains showing a reduced interlayer spacing of 3.1 Å and diamond domains with numerous stacking faults, with well defined interfaces between the two phases. Alternating red and cyan lines delineate the end-to-end connectivity between one atomic layer in graphite and kinked carbon bilayer in diamond traversing multiple graphite and diamond domains. c, d, Magnified HAADF-STEM images corresponding to the blue-boxed (c) and green-boxed (d) regions in b. The red and cyan lines and circles highlight the one-to-one correspondence between the atomic layers in graphite and the kinked carbon bilayers in diamond, respectively.

Fig. 3. Coherent interface structures between graphite and diamond.

a, b, Atomic-resolution HAADF-STEM images of two gradia interfaces (left) and the corresponding atomic models (right). The red and cyan lines (circles) delineate the one-to-one correspondence between graphite and diamond. In the atomic models, adjacent graphitic layers are coloured with different greyscales for clarity. Structural motifs at the interface are denoted with rhombi (with or without shadows) and rectangles (with different orientations). c, Four representative gradia interfaces. The pink and green sides in patterns indicate side lengths of 2.18 Å and 2.06 Å, respectively. See main text for details of nomenclature.

Fig. 4. Energy barriers and transformation process from graphite to diamond through intermediate crystals (hypothetic) containing gradia interfaces.

a, Energy profile of graphite-to-diamond transformation through different pathways at 10 GPa. The maximum energy barrier occurs when the wavy graphitic layers start bonding, that is, forming the gradia interface along HG to intermediate crystal. TS represents the transition states of the pathway from graphite to intermediate crystal; TS’ represents the transition states of the pathway from intermediate crystal to diamond. b, Energy barriers decrease with increasing pressure from HG to intermediate crystal. The classic concerted transformation pathways previously proposed—that is, HG→CD and HG→HD (A-path, R-path, O-path)—are from ref. . c, Energy barriers decrease with increasing pressure from intermediate crystal to CD (or HD). The energy barrier in this stage (diamond growth) is significantly lower than that in the nucleation stage (gradia interface formation). Above 10 GPa, Gradia-CO and Gradia-HB crystals can convert into diamond with almost no energy barrier. d,e, The structure snapshots during graphite-to-diamond transformation through Gradia-CO (d) and Gradia-HB crystals (e) at 10 GPa. The adjacent graphitic layers are distinguished with grey and blue colours. Under pressure, the graphite layers bend wavily. Bonding across graphite layers starts in the green-shadowed regions with a reduced interlayer spacing of about 2.1 Å, whereas the interlayer spacing increases from 2.9 Å to 3.2 Å in the yellow-shadowed regions, leaving graphite stable. Next, the interfaces advance gradually into graphite, and diamond nuclei eventually grow into pure diamond. The angles in the green- and yellow-shadowed regions indicate the localized changes in the structure.

Extended Data Fig. 1. Typical microstructures and Rietveld refinement analysis of samples recovered from 15 GPa and various temperatures.

a, 1,200 °C. b, 1,400 °C. c, 1,600 °C. d, 2,000 °C. With increasing synthesis temperature, the graphitic regions (enclosed with red curves) in the recovered samples decreases gradually. All scale bars are 20 nm. e−h, Rietveld refinement for different samples. Black bars: 'compressed graphite'; cyan bars: HD; orange bars: CD. i, Mass fraction of graphitic component in the samples with increasing synthesis temperature.

Extended Data Fig. 2. Atomic-resolution HAADF-STEM images of various gradia interfaces.

a–f, The average interlayer distances in graphite regions are 3.13 Å (a), 3.11 Å (b), 3.20 Å (c), 3.02 Å (d), 2.93 Å (e), and 3.09 Å (f). Red and cyan lines/circles indicate the one-to-one coherence between the atomic layers in graphite and the kinked carbon bilayers in diamond. Different stacking modes in diamond regions are emphasized with rhombi and rectangles (CD and HD motifs), respectively. All scale bars are 0.5 nm.

Extended Data Fig. 3. The advance of gradia interfaces into graphite.

a, Several new Gradia-CO motifs (open red rhombi) advancing into graphite one step further from initial Gradia-CO interface (filled magenta rhombi) shown in Fig. 3c. b, Several new Gradia-HC motifs (open red rectangles) advancing into graphite one or more steps further from initial Gradia-HC interface (filled cyan rectangles) shown in Fig. 3c. c, d, The interface propagation with extra bonding under electron-beam irradiation during STEM observation. The LAADF-STEM images captured at the same area illustrate atomic bonding at the interface. The white dotted circles mark the C−C dumbbell units in diamond, and the red ones correspond to the newly formed C−C units in diamond at the interface. All scale bars are 0.5 nm.

Extended Data Fig. 4. Comparison of different carbon structures.

a, The originally defined diaphite, that is, a bilayer structure with one third of atoms (coloured in red) forming bonds between layers, which is referred to as '2D diaphite' hereafter, has neither graphite unit nor diamond unit in the structure (adapted with permission from ref. , Fig. 5c. Copyright 2009 by the American Physical Society). b, The originally defined 2D diaphite structure used for surface adsorption propertie calculation, yellow carbon atoms indicate extruded sites for hydrogen adsorption (adapted with permission from ref. , Fig. 1. Copyright 2017 by Elsevier B.V.). c, Model structures for type 1 and type 2 diaphite. For type 2 diaphite, graphite and diamond regions are also connected via bonded interfaces, but without a one-to-one correspondence: depending on the choice of diamond {111} planes, 4 graphene layers are corresponding to 6 or 3 diamond (111) planes (see the interface in the red dashed circle in c) (adapted from ref. , Fig. 3a and 3b. CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)). d, Four structural models for Gradia-CO, Gradia-CA, Gradia-HB, and Gradia-HC with fully coherent interfaces, that is a one-to-one correspondence in atomic positions between graphite and diamond layers across the interface. They show different tilting angles between graphite and diamond layers and different interlayer spacings in graphitic regions. These newly found structures are clearly different from the 2D diaphite, type 1, and type 2 diaphite.

Extended Data Fig. 5. Hypothetical crystal structures with characteristic gradia interfaces and the corresponding electronic structures.

a, Gradia-CO. b, Gradia-CA. c, Gradia-HB. d, Gradia-HC. The hypothetical crystals are named after the gradia interfaces shown in Fig. 3c. For each of the crystal models displayed, the thickness of the cell perpendicular to the page is the lattice constant of graphite [010]_HG (or [110]_CD and [010]_HD). Details of structural information are listed in Extended Data Table 1. In each crystal structure, the grey-line grid indicates the unit cell employed for transformation energy barrier calculation, the cyan-shadowed area indicates the primitive cell. Carbon atoms are differently coloured: grey for sp²-hybridized atoms in graphitic sections, gold for sp³-hybridized atoms in diamond-structured sections, and green for those in gradia interfaces (among the interface atoms, atoms circled in red are sp²-hybridized, the others are sp³-hybridized). The lower part of each panel shows the calculated electronic structure with the bands across the Fermi level coloured in blue. In each DOS graph, the partial DOS (in unit of states per eV per atom) from interface, graphitic and diamond-structured atoms are coloured in green (int.), grey (gra.) and gold (dia.), respectively. Substantial contributions from the interface atoms to the density of states (DOS) near the Fermi level are revealed, especially for Gradia-CO and Gradia-HB crystals (a and c) with sp²-hybridized atoms at the interface. For instance, the electronic structure and partial density of states (PDOS) of Gradia-CO demonstrates a clear metallicity because of several electron bands across the Fermi level. The electronic states around Fermi level mainly come from the p_x and p_z orbitals from two sets of carbon atoms: C1 atoms (sp²-hybridized ones at the interface, red-circled) and C2 atoms (at the zigzag edge of graphene layer connecting the interface).

Extended Data Fig. 6. The ambient-pressure band structures of four Gradia structures calculated by DFT-PBE and HSE06 functional as implemented in VASP code.

There are bands across the Fermi level, indicating metallic Gradia-CO, Gradia-CA, Gradia-HB, and semimetallic Gradia-HC at ambient pressure.

Extended Data Fig. 7. Thermodynamic, mechanical, and dynamic stabilities of four crystal structures shown in Extended Data Figure 5.

a, Formation enthalpies with respect to graphite as a function of applied pressure. Crystals containing gradia interfaces become stable energetically relative to graphite at high pressures in the range of 10−20 GPa. Note that Gradia-CO and Gradia-HB transform directly into CD and HD above 11 GPa and 19 GPa, respectively. This might be related to the high-energy sp²-hybridized atoms at gradia interfaces, which can contribute to diamond growth by advancing the interface towards the graphite side under pressure. b, Calculated elastic constants (C_ij, GPa), bulk moduli (B, GPa), shear moduli (G, GPa), and Young's moduli (E, GPa) of hypothetical crystal structures at ambient pressure. Clearly, the calculated elastic constants C_ij of crystal structures satisfy the mechanical stability criteria, confirming their mechanical stability at ambient pressure. c, Calculated phonon spectra of the hypothetical crystal structures at ambient pressure. No imaginary phonon frequencies throughout the whole Brillouin zone indicates that all the structures are dynamically stable.

Extended Data Fig. 8. Transformation process from graphite to diamond through Gradia crystals.

a, Energy profile of transformation from graphite to Gradia-CO structures with different unit cell size or different graphite and diamond fractions at 10 GPa. b, Nomenclature of Gradia-CO (m, n): m is the number of six-numbered rings to be formed between adjacent layers in graphite domain, and n is the number of six-numbered rings formed in diamond domain. Red dotted lines indicate further bonding to six-numbered rings for a complete transformed diamond. c, d, Structure snapshots during graphite-to-diamond transformation through Gradia-CA and Gradia-HC crystals, respectively, at 10 GPa.

Extended Data Fig. 9. Mechanical and electrical properties of Gradia samples.

a, Knoop hardness (H_K) as a function of applied loads for Gradia samples quenched from 15 GPa/1,200 °C (black), 15 GPa/1,600 °C (turquoise), and 15 GPa/2,000 °C (red). Error bars represent one s.d. (n = 5). H_K of three samples under 9.8 N load are 51 ± 4.7, 69 ± 6.0, and 115 ± 9.3 GPa, respectively. In comparison, H_K of binderless nanopolycrystalline diamond (NPD) under 9.8 N is 111 ± 15.2 GPa. b, A photograph of black Gradia sample with a polished surface (15 GPa/2,000 °C). c, Vickers indentation fracture toughness of Gradia (15 GPa/2,000 °C), binderless nanopolycrystalline diamond (NPD) and single-crystal diamond (SC-D, (111) face)_. The estimation of fracture toughness of materials is based on the length of cracks generated on the sample with a Vickers indenter of square-pyramid diamond under high loads of 49 N (for Gradia and NPD) and 19.6 N (for SC-D). All scale bars are 20 µm. Unlike other materials with obvious cracks, no visible crack was generated in Gradia, indicating an excellent toughness of Gradia. The fracture toughness is 8.7 ± 1.8 MPa·m^0.5 for NPD and 6.4 ± 1.1 MPa·m^0.5 for SC-D. d, Temperature-dependent electrical resistivities of Gradia samples, showing a semiconducting characteristic. With increasing synthesis temperature, the diamond content in Gradia increases gradually, resulting in an increase in electrical resistivity. Gradia samples recovered from 15 GPa/1,200 °C and 15 GPa/1,600 °C show low resistivities, while the sample recovered from 15 GPa/2,000 °C shows a significantly high resistivity. The mechanical and electrical properties of Gradia can be tunable by adjusting the proportions of graphite and diamond in the bulk.