First-principles Studies Of Structures And Properties Of Multifunctional Material BC₇

Superhard materials are of great interest because of their widespread applications in cutting, abrasives, and hard coating. They usually have strong covalent covalent bonds, high valence-electron density, large modulus, and large strength. Some of them are consist of strong covalent compounds of B, C, and N, such as diamond, c-BN, and BC2N. Up to now, diamond has been successfully used in electronics, optics, material science, and surgery. However, there exist limitations to its applicaibity. It is easy to react with oxygen and ferrous metal during the process of machining. Therefore, more thermally and chemically stable novel diamondlike phases are motivated by real applications of superhard materials.Because B-C materials are more resistant to oxygen and ferrous metals, the diamondlike B-C compounds attract the attention of many researchers. On the other hand, superconductivity was discovered in boron-doped diamond, which has been proved to have very high Tc with large boron content. Ultra hardness combined with special electronic properties promise a wide variety of applications of diamondlike B-C compounds. Recently, Solozhenko et al. successfully synthesized c-BC5, which is a classical semiconductor with high Vickers Hardness (71 GPa), and large bulk modulus (335 GPa). This result is a little different from the theoretical result predicted as a superconductor with 45 K of Tc by Calandra et al. afterward. Forthermore, according to following investigations on more complex structures, there is a debate on the configuration of c-BC5. So, developing a stable B-C compound is a scientific problem requiring more and deeper and detail research. Therefore, in order to carry out the structures, stabilites, properties, and its machnism of such category of material, we introduce more than a dozen possible configuration of BC7 and perform a systematic investigation on the structures and properties of BC7 by first-principles calculations. We found that o-BC7 (base-centered orthogonal symmetry, space group Amm2) is the most stable one of all the structures. Our further calculated results indicate that o-BC7 is a superhard material with hardness of 49.7 GPa, at the same time it is also a superconductor with Tc of 38 K. The strong electron-phonon coupling is attributed to the introduction of B elements, which brings a large density of states upon Fermi level, and leads all collebrate vibrations of carbon atoms.