First-principles study of hypothetical boron crystals: Bn(n=13, 14, 15)

First-principles simulations within density functional theory are performed to investigate structural, electronic and mechanical properties of hypothetical boron crystals B-n(n = 13, 14, 15). These hypothetical crystals are generated by inserting boron atom(s) to the space in three-dimensional network of alpha-boron (alpha-B-12). The effects of inserted atom(s) and their site(s) on the lattice parameters, mechanical and electronic properties are discussed. Cohesive energies and formation enthalpies are calculated to discuss energetic stability of purposed compounds, and also the elastic constants are determined to study mechanical stability and mechanical properties such as bulk, shear and Young moduli. To check the phase stability, molecular dynamics simulations and transition state search calculations are performed and to emphasize distinction of the phases energy-volume curves for all phases are presented. From calculated density of states and Mulliken atomic charges/bond overlap populations, it is observed that the charge transfers exist between inserted boron atom(s) located at different sites and icosahedral boron atoms. By mean of the optimized ground state geometry and other first-principles results, the micro-hardnesses of each boron phases are calculated. (C) 2012 Elsevier Masson SAS. All rights reserved.