Evolutionary algorithm based structure search and first-principles study of B12C3polytypes

Abstract

The structure search based on evolutionary algorithm has yielded six unique Boron Carbide structures in B<inf>12</inf>C<inf>3</inf>stoichiometry, three of them with negative formation energies. Their formation energies lie within a band of 166 meV/atom, so they can be formed together in any optimal high temperature synthesis of B<inf>12</inf>C<inf>3</inf>and they are thermodynamically stable at temperatures up to 660 K. This work is the first independent confirmation using structure search that B<inf>11</inf>Cp(CBC) is the ground state structure of B<inf>12</inf>C<inf>3</inf>stoichiometry. New structures like the 14-atom-cage and the supercell (B<inf>11</inf>Cp)(B<inf>10</inf>Cpe <inf>2</inf>)(CBC)(CBB) have also emerged in the structure search. Five structures have base-centered monoclinic symmetry and the supercell has triclinic symmetry, implying that the determination of monoclinic symmetry in B<inf>12</inf>C<inf>3</inf>by experimental measurements is an option for further inquiry. The mechanical stability of these systems are established through the analysis of their elastic constants and their dynamical stability from the phonon data. The high value of Bulk modulus (?250 GPa) indicates their high hardness and the B/G value confirms their brittle nature. The electronic structure shows that they are semiconductors with a significant reduction in the band gap when the structure does not contain the CBC chain. The curve fitting of the cumulative IR spectrum against the experimental spectrum implies that the presence of B<inf>11</inf>Cp(CBC) in the ground state composition could mostly be through structures of larger unit cells. The hardness values of these systems estimated by using the semi-empirical model based on bond strength are in excellent agreement with the experimental values. For the four structures with chain the hardness values are close to the superhard regime (>40 GPa). © 2016 Elsevier B.V.