Nucleation mechanism for the direct graphite-to-diamond phase transition

Abstract

Graphite and diamond have comparable free energies, yet forming diamond from graphite is far from easy. In the absence of a catalyst, pressures that are significantly higher than the equilibrium coexistence pressures are required to induce the graphite-to-diamond transition [1][2][3][4][5][6][7] . Furthermore, the formation of the metastable hexagonal polymorph of diamond instead of the more stable cubic diamond is favored at lower temperatures 2,5-7 . The concerted mechanism suggested in previous theoretical studies 8-12 cannot explain these phenomena. Using an ab initio quality neural-network potential 13 we performed a largescale study of the graphite-to-diamond transition assuming that it occurs via nucleation. The nucleation mechanism accounts for the observed phenomenology and reveals its microscopic origins. We demonstrated that the large lattice distortions that accompany the formation of the diamond nuclei inhibit the phase transition at low pressure and direct it towards the hexagonal diamond phase at higher pressure. The nucleation mechanism proposed in this work is an important step towards a better understanding of structural transformations in a wide range of complex systems such as amorphous carbon and carbon nanomaterials.Static compression of hexagonal graphite (HG) results in the formation of metastable hexagonal diamond (HD) at temperatures around 1200-1700 K 2,5-7 and cubic diamond (CD) at higher temperatures 1,[3][4][5]7 . Although the transition pressure is sensitive to the nature of the graphite samples neither of the diamond phases has been observed to form below ∼12 GPa. This pressure is significantly higher than the graphite-diamond coexistence pressure approximated by the Berman-Simon line P (GPa) ∼ 0.76 + 2.78 × 10 −3 T (K) 14 .Despite being an area of intense theoretical research 8-12 the microscopic mechanism of the formation of metastable HD and the reason for the remarkable stability of graphite above the coexistence pressure are still unknown. Computer simulations, which could help resolve these issues, have been hindered because of the inability of empirical potentials to describe the energetics of the transformation accurately 13,15 and the computational expense of more reliable ab initio methods. In the latter case, short simulation time and small system size (i.e. several hundred atoms) force the transition to occur in a concerted manner with the ultrafast (∼ 10 −2 − 1 ps) synchronous formation of all new chemical bonds accross the entire simulation box 11,12,16 . While concerted mechanisms can be observed at shock compression 16-18 , the transformation under static conditions is expected to proceed via nucleation and growth.It has been estimated that because diamond has an extremely high surface energy 19 its critical nuclei may contain thousands of atoms 20-22 . Hence, tens or even hundreds of thousands of atoms are required for modeling the diamond nuclei and the surrounding graphite matrix. Direct ab initio simulations of systems of this size are outright impossible. Ther...

Electronic transition from graphite to graphene via controlled movement of the top layer with scanning tunneling microscopy

“…The two systems eventually begin to interact, and the graphene transitions to a layer of graphite. Interestingly, if pressure were applied still further, a second transition would occur from graphite to diamond, 37 as has been recently verified experimentally using femtosecond laser pulses to achieve the change. 38 However, what makes the graphene to graphite transition special is that it is the only known system where one can observe with atomic resolution how the electron acquires mass; or alternatively, how the electron loses mass and graphene generates the giant charge density responsible for its high current carrying capacity and thermal conductivity.…”

Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material

“…Observations and theoretical studies suggested a structural relationship among graphite, cubic diamond and lonsdaleite and an important role of the latter during the graphite-todiamond transition 3,[9][10][11][12][13][14] . Furthermore, an area centred around 18 GPa and 1,400 K in the pressure-temperature diagram for carbon was attributed to a phase called 'retrievable hexagonaltype diamond' 10 , which corresponds to the conditions where lonsdaleite has been reported 3,11,15 .…”

Nonclassical Nucleation in a Solid-Solid Transition of Confined Hard Spheres

“…Solid-solid transitions usually occur in a martensitic fashion [17,18] involving the concerted, diffusionless motion of the atoms in the unit cell. Anisotropic stress, rapid quenching, and a small system size have been found to promote martensitic transformations [19]. In colloids, martensitic transitions have been observed in small crystalline clusters [20][21][22] or lattices stretched by external fields [18,[23][24][25].…”