The paper was published in Diamond and Related Materials (2022)
Many carbon phases exhibit attractive properties, which further motivates researchers to continue the search for new stable compounds. Some experimental findings suggest the possible existence of an intermediate carbon phase (ICP) between diamond and graphite. Intermediate structures were observed in the process of diamond formation, where products, obtained by high-temperature shock compression of wood charcoal besides nanocrystalline diamond contained an amorphous carbon phase having a density intermediate between the densities of the original charcoal and diamond. In the presence of water, other mechanisms of the diamond-graphite transition with the formation of other intermediate states are possible with a formation of metastable phase of a linear hydrocarbon.
In the present work we report the successful synthesis and investigation of physical properties of the intermediate carbon phase. For the first time, this phase was obtained by the treatment of diamond in the planetary mill. Under conditions of cyclic stresses near the graphite-diamond boundary at the carbon phase diagram at low temperatures the following processes occur: plastic deformation by the mechanical twinning; martensitic phase transition of diamond to ICP; transition of ICP to onions with a radius below ~5 nm.
Figure (a) shows a particle containing not only a diamond but also a fragment of ICP crystal lattice with an interplanar distance of 0.255 nm which forms an angle of 13º with planes (002) of the diamond. This data proves that the diamond was not broken completely but transformed into a new structure and therefore in the process of transformation of the plane (111) of diamond into the plane (001) of graphite or vice versa, the intermediate carbon phase structure is formed.
a) Inverse Fourier transformation. The diamond structure is broken, but it does not change into graphite. Planes with an interplanar spacing of 0.255 nm appear, which form an angle of 13º with planes (002) of the diamond b) side view of the proposed model for ICP with sp3-hybridized atoms concentration 1.6 %. The unit cell is marked by the dashed line. In the inset the enlarged region of crosslink is shown. Vacancy surrounding atoms and interstitial atoms (crosslinks) are marked by green and orange colors, respectively; c) Energy vs volume for the graphite, diamond and ICP with various sp3-hybridized atoms concentration; d) Enthalpy differences of ICP and diamond relative to graphite vs. pressure. In (c,d) data for graphite and diamond are marked by black hexagons and diamonds, respectively, ICP with sp3-hybridized atoms concentration of 1.6, 2.0, 2.8, 4.0, 6.3, 11.1 and 43.8 % are marked by red, orange, yellow, green, cyan, blue, and purple circles, respectively
We proposed that the intermediate interlayer distance between graphite and diamond originates from the crosslinking of sp2-hybridized layers by self-interstitial (Frenkel) defects (Figure (b)). In the proposed model the intimate Frenkel pair defects are located on top of each other throughout the structure.
We consider the energy–volume calculations and the thermodynamic criterion of equal free energies to study the stability of the ICP phase comparable with diamond and graphite. These phases are equilibrated at several volumes and their energy–volume relations are fit to the third-order Birch–Murnaghan equation of state. We estimated bulk moduli values using this equation. It was found that all ICP phases display similar B0 values between 240 and 300 GPa, close to graphite value 258 GPa. The computed total energy as a function of volume is presented in Figure (c).
It was found that the total energy is lower than diamond for the five concentrations of sp3-hybridized atoms until 6.3%. The structures at these concentrations are relatively sparse, with crosslinks separated from each other by 1.97, 1.73, 1,48, 1.25 and 0.99 nm, respectively. Meanwhile, the interlayer distance in the vicinity of crosslinks was found to be 0.26 nm which corresponds well to experimental data.
The covalent bonding of such concentration prevents the layers from separation and does not distort the structure too much. These structures are the most consistent candidates for describing the phase obtained in the experiment. For the lower concentration (less than 1.6 %), the van der Waals repulsion transforms ICP into a graphite structure whereas a higher concentration of crosslinks buckles the layers with inducing the mechanical strain and rises the overall strain energy.
The enthalpy analysis allowed us to conclude that under pressure most favorable ICP phase (sp3-hybridized atoms concentration 1.6 %) become stable with respect to graphite beyond 8 GPa (Figure (d)). We can therefore assume that this phase may be a side product of some of the pressure-induced metastable phase transformations of graphite. On the other hand, the high-density structure (sp3-hybridized atoms concentration 43.8%) becomes more stable only after 70 GPa.
The covalently bound crosslink enhances the out-of-plane stiffness of the structure. We found that at a crosslink concentration range from 1.6 to 6.3 % C33 constant slightly varies about 200 GPa whereas the denser distribution of crosslinks yields C33 value of 358 GPa (x = 44 %). These values are located between graphite and diamond values 49 GPa and 1079 GPa, respectively. Another important property of the phase is the appearance of out-of-plane conductivity. The density of electronic states of all considered ICP structures displays finite states on the Fermi energy which justifies the metallic properties consistent with the observed drop in resistance of irradiated graphite systems
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