Superhard and superconductive polymorphs of diamond-like BC3

We have explored the crystal structures of synthesized diamond-like BC3 (d-BC3) with particle swarm optimization (PSO) algorithm combined with first-principles structural optimizations. Three intriguing metallic low-energy structures are uncovered: (i) orthorhombic Pmma-a consisting of a sandwich-like B-layer between C layers, (ii) orthorhombic Pmma-b with novel B-B bondings, and (iii) tetragonal P-4m2. The simulated Raman modes of Pmma-b phase are in agreement with experiments. Further hardness and electron-phonon calculations revealed that all the three candidate d-BC3 phases are superhard (> 40 GPa) and superconductive materials with the superconducting critical temperature reaching at 16.6-23.4 K for Pmma-a phase. (C) 2010 Elsevier B.V. All rights reserved.