New Crystal Structures of IrB and IrB2: First-Principles Calculations

Superhard IrB1.35 [Chem. Mater. 2009, 21, 1407] and IrB1.1 film [ACS Appl. Mater. Interfaces 2010, 2, 581] have been synthesized in experiment, but the structural formulas of iridium borides with integral ratio between Ir and B atoms are still undefined up to now. Here, we use a combination of particle-swarm optimization technique and first-principles calculations to explore the crystal structures of IrB and IrB2. We demonstrate that the new phase P-1-IrB belongs to the orthorhombic Pnma space group, while P-5-IrB2 (space group Pmmn) has a same structure type with OsB2. At the pressure of about 5 GPa, a phase transition occurs between the Pnma and anti-NiAs phases for P-1-IrB. Further phonon and elastic constants calculations imply that both P-1-IrB and P-5-IrB2 are dynamically and mechanically stable and are potential low compressible materials because of their high bulk moduli. The analysis of density of states and chemical bonding indicates that the formation of strong covalent bonding in these compounds contributes greatly to their stabilities.