First Principle Predictions Of High Density Phases Of BC₂N And B₂CN

High density phases of ternary B-C-N compounds were systematically studied by using density functional theory of first principle.Using crystallographic topology, the seven kinds of the possible high density BC2N phases were constructed by using diamond unit cell with an eight-atom zinc-blende structure as prototype. The parameters of crystal structure at ground state were obtained after full geometry optimization by using BFGS method. It is found that monoclinic BC2N-V is energetically lowest , orthorhombic BC2N-I and BC2N-II are slightly larger in seven possible structures by calculating their total energies, cohesive energies and formation energies at ground state. These three structures in seven structures are more stable than c-BN. The calculation for bulk and shear modulus show that the shear modulus of BC2N-I and BC2N-II are smaller than that of c-BN. It is found that all BC2N structures have energy gaps, which indicates that they are semiconductors. Most stable BC2N-I and BC2N-II demonstrate direct energy gap.The same method was used to construct and study the high density phases of ternary B2CN. The parameters of crystal structure of B2CN were obtained at ground state. The studies on the stability of B2CN show that B2CN-III is most stable, but still less than c-BN. The calculations for bulk and shear modulus indicate that the shear modulus of only B2CN-III is slightly less than that of c-BN, and still less than those of BC2N-I and BC2N-II. The study on the electronic properties of B2CN convinces that B2CN is not belong to semiconductor, and B2CN-III is a kind of new compound with hopping conductivity...