Crystal Structures Design Of New B-C-N-O Superhard Materials Under High Pressure

The search on new superhard materials is of great importance in view of its major roles played for the fundamental science and the industrial applications. Recent experimental synthesis has made several great successes, but the synthetic difficulty in general remains. Materials design technique is greatly desirable as a request to assist experiment. On search for new superhard materials over the past several decades, scientists mainly focused on the exploration of covalent compounds formed by light elements, namely, boron (B), carbon (C), nitrogen (N), and oxygen (O). since these elements have the ability to form short and strong three dimensional covalent bonds (e.g., sp3 bonds), which is a necessary condition for superhard materials. Very recently, a new family of materials formed by heavy transition metals and light elements are proposed to be potential superhard since heavy transition metals can basically introduce high valence electron density into the compounds to resist both elastic and plastic deformation. Many these compounds have then been successfully synthesized, such as. transition metal nitrides, carbides, and borides. These compounds possess ultrahigh bulk moduli (428 GPa for IrN2]) comparable with those of the traditional superhard materials. Though there exists extensive debates,ReB2 and WB4 have been claimed to be superhard.Earlier experiments indeed have made big progress on synthesizing several new superhard materials, but the well-known synthetic difficult remains. The blind synthesis of superhard materials requires a lot of different tries, such as the choice of precursor materials, synthetic temperature and pressure, etc., which normally need a large amount of manpower and materials resources. However, it is quite often that the synthesis faces little rate of success, as well as low efficiency. One of the causes might be from that some materials can only be stabilized in a very narrow temperature and pressure regime. Therefore, there is an urgent need for robust methods on designing new superhard materials. Once a promising energetic phase has been predicted and the appropriate synthetic conditions (e.g., external pressure, temperature and starting materials) could be suggested to help for the experimental synthesis in a more effective way.Based on the fact that the most stable crystal structure has the lowest Gibbs free energy at given P-T conditions, several structural prediction methods performed without any prior knowledge or assumptions about the system, such as evolutionary methodology, particle swarm optimization (PSO). simulated annealing, minima hopping, and metadynamics. have been developed to predict the stable crystal structures. The evolutionary method and particle swarm optimization for crystal structure prediction have been very successful to explore the stable crystal structures with the only known information of chemical compositions. It is accepted that the energetically most stable phases (or some low-energy metastable phases) of the target materials are more likely to be synthesized with carefully chosen experimental conditions (if possible, at the theoretically suggested pressure and temperature region). Here we present some applications of the evolutionary algorithm and particle swarm optimization on design of superhard materials targeting on the technically important systems, such as the compounds formed by light elements or by heavy transition metals and light elements.Carbon can adopt a wide range of structures, such as graphite, diamond, hexagonal diamond (lonsdaleite), carbynes. nanotubes. fullerences, and amorphous carbon. This is because of carbon's ability to form sp. sp2, and sp3 hybridized bonds. Under high pressure, carbon and its symmetrical analogs exhibit a tendency to form strong directional bonds, especially when the electronegativity difference between the two bonding atoms is small (e.g., diamond and c-BN). We have extensively explored the crystal structures of elemental carbon under pressure (0～100 GPa). Remarkably, a novel monoclinic phase (named as M-carbon) with C2/m symmetry was uncovered to be stable over graphite above 13.4 GPa. The crystal is made of exclusively three-dimensional spJ hybridized covalent bonds, just as in the well-known (2×1) reconstruction of the (111) surface of diamond and silicon. Since M-carbon presents six-fold rings forming warped "layers", this intriguing phase can be understood as distorted graphite. Strikingly, this new polymorph of carbon possesses very high hardness of 83 and bulk modulus of 431 GPa, which are comparable to those of diamond. Experimentally, it is known that graphite can convert to a superhard unknown phase above 14 GPa at room temperature. We here proposed that M-carbon is a likely candidate for this cold-compressed graphite, since the experimentally observed changes in X-ray diffraction pattern, near K-edge spectroscopy. and electrical resistance of this superhard phase are well explained by the coexistence of M-carbon and graphite。We have extensively explored the crystal structure of BC2N using ab initio evolutionary methodology. We have predicted three polytypic structural families: orthorhombic Pmm2-nu. hexagonal P3m1-nu. and rhombohedral R3m-nu. Analysis of the total energy, simulated X-ray diffraction pattern and energy-loss near-edge spectroscopy suggests that our predicted R3m-2u is the best candidate phase for the observed superhard BC2N. We have also demonstrated that the previously proposed high density and low density forms might be identical and their X-ray diffraction patterns could be reasonably understood by the single phase of R3m-2u. The estimated theoretical Vichers hardness [1] of R3m-2u BC2N is 62 GPa It is significant to note that the hardness of R3m-2u BC2N exceeds than that of c-BN in this calculation, which is consistent with the experimental result. Recently, diamond-like c-BC5 with high B content has been successfully synthesized. In order to identify the experimentally synthesized phase and uncover other new superhard phases, we investigated BC5 at a wide pressure range of 0-100 GPa using the evolutionary algorithm and particle swann optimization. After examining the dynamical stability, the energetically most preferable polymorphs are two orthorhombic Pmma phases (Pmma-1 and Pmma-2). The simulated X-ray diffraction patterns. Raman modes of the two Pmma phases show remarkable agreement with the experimental data. The Vickers hardness of the two Pmma structures have been estimated to be 74 and 70 GPa. respectively, in satisfactory agreement with the experimental data (71 GPa). We propsed that for hole conductors, such as c-BC5, the major cabriers are holes and the valence electrons are mainly localized to form covalent bonds. It is thus unnecessary to include the metallic correction in the hardness calculation for c-BC5.An isoelectronic and asymmetrical compound of diamond. B2O, was experimentally synthesized by Endo et al. through the reaction of BP with oxygen under conditions of 2.0-6.0 GPa and 800-1350℃. The Vickers'microhardness of the sintered sample is in the range 33.5 to 40.5 GPa. Using ab initio evolutionary methodology and particle swarm optimization for crystal structure prediction, we have investigated the candidate crystal structures of the synthesized superhard B2O. A monoclinic phase C2/m of B2O was found to show the lowest enthalpy among the previous and current predicted structures and is an excellent potential superhard material with a simulated hardness of 66.7 GPa. Unfortunately, the XRD of C2/m does not fit into the experimental XRD data of the synthesized sample. From the simulated XRD patterns, we propose that the most likely structure for the synthesized superhard B2O is the P-4m2 or even a mixture of P-4m2 and P42mc, which are not diamond-like. By comparison with C2/m B2O, the fact of large positive energies of the thermodynamically metastable P-4m2 and P42mc structures could explain why the B2O sample is not easily synthesized later.Experimentally. OSB2, ReB2,and IrN2 have been demonstrated to have high bulk modulus of 365-395.360, and 428 GPa respectively, indicating those compounds possess excellent ultra-incompressibility. However, the hardness obtained from the asymptotic load-independent region of the hardness as a function of load for the synthesized TM-LE compounds is not high enough as excepted (<40 GPa). The theoretical calculations for these heavy transition metals' compounds draw a clear conclusion that the highly directional LE-LE bonds with large electron densities are short and strong; however, the TM-LE bonds with lower electron densities are long and weak. This anisotropic bonding behavior might be a severe problem for the hardness. In addition, the bonds in the compound formed by heavy transition metals and light elements typically have metallic component which is delocalized and might be negatively related to hardness. Thus, design of superhard phases in these compounds may pay particular attention to the nonmetallic materials with isotropic bonding environment. We thus densign element-encapsulated caged structures of carbon (M@C12 and M2@C12) as potential superhard materials. Electron-phonon coupling calculations show that Na@C12 (H2@C12) is superconducting with a high Tc of 23 (34) K.