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.
Highlights
The strength of super- or ultra-hard materials makes them important for a variety of applications such as, drilling, cutting, wear-resistant coating and abrasives
An ultra-hard material which is more suitable than diamond for machining ferrous materials is cubic boron nitride (c-BN), because it is more chemically inert to redox reactions with iron at high temperatures
Single crystal super-hard materials have been experimentally synthesized under high temperature and pressure conditions using laser heated diamond-anvil cells (DAC) (Stavrou et al, 2016)
Summary
The strength of super- or ultra-hard materials makes them important for a variety of applications such as, drilling, cutting, wear-resistant coating and abrasives. Ternary compounds of the boron-carbon-nitride (B-C-N) system like BC2N have attracted extensive researcher interest in the search for materials that are harder than cBN and chemically more stable than diamond at elevated temperatures. It remains a major challenge to determine the crystal structure of BC2N using experimental methods like x-ray diffraction (XRD) because of the very close and low values of atomic masses for boron, carbon and nitrogen atoms, which are 10.81, 12.01 and 14.01 respectively; these three elements areneighbors‘ on the periodic table and their diffraction peaks tend to overlap. First principles methods are important in determining the crystal structure of BC2N polymorphs. This knowledge could assist in developing cheaper methods of synthesizing the material. First principles methods may assist in exploring other technologically useful properties of BC2N
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