Abstract

Crystal lattice structure searching by Particle Swarm Optimization (PSO) and first-principles structural optimization have been used to explore polymorphs of BC2N, possessing sp3 hybridization, under a varying applied hydrostatic pressure. Two low Gibbs free energy structures were identified: one with a primitive orthorhombic structure and Space Group, Pmm2, and the other with a primitive tetragonal structure and Space Group, P m2. Dynamical and mechanical stabilities of the Pmm2, orthorhombic BC2N (o-BC2N) structure were established using its phonon dispersions and elastic constants. The bulk modulus of this predicted BC2N phase was 377.15 GPa, which indicates a super-hard compound. The material is brittle with a B/G ratio of 0.911 and a low degree of elastic anisotropy with a Universal Elastic Anisotropy Index of only 0.774%. Calculations of the electronic band structure demonstrated that the material is a direct band gap semiconductor with a band gap of 1.731 eV at zero applied pressure. The band gap increases monotonically with increased applied pressure and saturates to a value of about 1.756 eV above 1500 kbars; the hydrostatic pressure coefficients associated with this process were determined. 

Key words: High pressure phase stability, elastic anisotropy, ultra-hard material.