Decompression-Induced Diamond Formation from Graphite Sheared under Pressure

Graphite is known to transform into diamond under dynamic compression or under combined high pressure and high temperature, either by a concerted mechanism or by a nucleation mechanism. However, these mechanisms fail to explain the recently reported discovery of diamond formation during ambient temperature compression combined with shear stress. Here we report a new transition pathway for graphite to diamond under compression combined with shear, based on results from both theoretical simulations and advanced experiments. In contrast to the known model for thermally activated diamond formation under pressure, the shear-induced diamond formation takes place during the decompression process via structural transitions. At a high pressure with large shear, graphite transforms into ultrastrong $s{p}^3$ phases whose structures depend on the degree of shear stress. These metastable $s{p}^3$ phases transform into either diamond or graphite upon decompression. Our results explain several recent experimental observations of low-temperature diamond formation. They also emphasize the importance of shear stress for diamond formation, providing new insight into the graphite-diamond transformation mechanism.

Figure 1

(a) Optimized structure of hexagonal graphite hydrostatically compressed to 20 GPa. The lattice constants $oa$ ($ob$) and $oc$ are 7.3 and 11.3 Å, respectively. The interlayer spacing is 2.82 Å. (b),(c) Schematic illustration for the calculations on graphite under shear stress. During the simulation, the lattice is first sheared by rotating the $oc$ by an angle $\theta$ toward the $x$ axis (b). After this, a rotation perpendicular to the $z$ axis by an angle $\omega$ is applied for further shear operation. The blue rhomb in (c) represents the position of the top graphene plane of the graphite cell after the shear operation.

Figure 2

Two examples [(a)–(d) and (e)–(h)] of the transformation pathway for hexagonal graphite to diamond under 20 GPa with different shear stresses. (a),(b) The formation of a layered diamond structure ($C2/m$) under the shear operation $\theta =44°$, $\omega =90°$ at 20 GPa. (c) Snapshot of layered diamond during decompression. The lattice distorts and diamond layers bond into a fully $s{p}^3$-bonded structure. (d) Transformation into cubic diamond upon decompression to atmospheric pressure. (e),(f) The formation of an $s{p}^3$ carbon structure with $P\text{−}1$ symmetry group under the shear operation $\theta =45°$, $\omega =0°$. (g) Snapshot of the fully $s{p}^3$-bonded structure during decompression. (h) Transformation into cubic diamond when pressure is released.

Figure 3

(a) Pressure at the indenting point of the two NG spheres, measured from the high-frequency edge of the diamond Raman line, as a function of the chamber pressure. The inset shows a schematic illustration of the uniaxial compression experiment. (b) Finite element simulation of the shear distribution in the $L$ sphere under uniaxial compression at a confining pressure of 50 GPa. Raman spectra of (c) the $L$ sphere and (d) the $S$ sphere recorded at selected pressures upon compression. A 514.5 nm laser was used for excitation. (e) The corresponding pressure dependence of the $G$-band frequencies for the two NG spheres.

Figure 4

HRTEM images of the decompressed $L$ sphere, showing the areas containing graphite nanocrystals (a) and diamond nanograins (b). The inset in (b) gives the corresponding diffraction pattern; the angle between the reflexes is about 70°, which is consistent with that between the (111) planes of cubic diamond. (c) HRTEM image of the recovered $S$ sphere showing only nanocrystalline graphite. (d) EELS spectra of decompressed NG spheres measured from the areas indicated in (a)–(c) and a pristine NG sphere.