First-principles Study Of Sp~3Bonding Carbon And BN Polymorph

Static compression of graphite is an interesting way to obtain cubic diamond (3C)and hexagonal diamond (2H). It usually results in2H at temperatures from1200K to1700K and about15GPa, while the3C will be produced at high temperatures from1600K to2500K. Other diamond polymorphs, such as the4H,6H,8H,9R,12R,15R and21R,can be formed only under proper conditions. Besides, there is not enough energy toconquer energy barriers between graphite and2H or3C, an unknown superhard phase isproduced by cold compression of graphite. The crystal structure of this new phase is notwell determined. Recently, theoretical computations have proposed many possiblestructures for the cold compression graphite phase as candidates, such as M-carbon,Z-carbon, W-carbon, H-carbon, S-carbon and bct-C4. Besides, the BN polymorphs, suchas c-BN, w-BN, bct-BN (Pct-BN) and Z-BN (O-BN), have also been forecast andanalyzed.Diamond polymorphs can be regarded as the different stacking combination by threetypes of the graphene layers including chair type, ship type and distorted ship type bystudying the crystal structures of diamond polymorphs. While BN polymorphs can beregarded as the different stacking combination by two types of the h-BN layers includingchair type and ship type. The BN polymorphs contain only even rings due to theboron-boron (B-B) and nitrogen-nitrogen (N-N) bonds can’t exist stably. In this work, webuild diamond polymorphs (chair and ship type) and BN polymorphs (chair and ship type)based different graphene layers and h-BN layers. We study the influence of temperature onthe phase transition of diamond and BN polymorphs. Besides, the phase stability,mechanical and electrical properties of diamond and BN polymorphs has also been studied.For the diamond and BN polymorphs of chair type, we found that2H with3C and w-BNwith c-BN can coexist respectively at certain temperature range. Those diamond (BN)polymorphs are mechanical stability and may be the metastable phase of graphite (h-BN)at high temperature and high pressure. But the diamond (BN) polymorphs of ship typemay be the phase of cold compression graphite (h-BN). These diamond and BN polymorphs of the shear modulus, elastic modulus, Young’s modulus and hardness aresimilar to diamond and BN. The calculation results and the segment combination methodprovide a new way for the design and synthesis of new superhard material.