The ground-state structure and physical properties of ReB3 and IrB3 predicted from first principles

Abstract

ReB₃ has been synthesized and was reported to have symmetry of P6₃/mmc [Acta Chem. Scand. 1960, 14, 733]. However, we find that this structure is not stable due to its positive formation energy. In 2009, IrB_1.35 and IrB_1.1 were synthesized and were considered to be superhard [Chem. Mater. 2007, 21, 1407; ACS Appl. Mater. Interfaces 2010, 2, 581]. Inspired by these results, we explored the possible crystal structures of ReB₃ and IrB₃ by using the developed particle swarm optimization algorithm. We predict that Pm2-ReB₃ and Amm2-IrB₃ are the ground-state phases of ReB₃ and IrB₃, respectively. The stability, elastic properties, and electronic structures of the predicted structures were studied by first-principles calculations. The negative calculated formation enthalpies for Pm2-ReB₃ and P6₃/mmc-ReB₃ indicate that they are stable and can be synthesized under ambient pressure. Their dynamical stability is confirmed by calculated phonon dispersion curves. The predicted P6₃/mmc-ReB₃ has the highest hardness among these predicted structures. The calculated density of state shows that these predicted structures are metallic. The chemical bonding features of the predicted ReB₃ and IrB₃ were investigated by analyzing their electronic localization function.