Phase diagram and physical properties of iridium tetraboride from first principles.

Using both the swarm-intelligence-based CALYPSO method and the multi-algorithm-collaborative (MAC) algorithm for crystal structure prediction, three unexpected new phases (P63/mmc, C2/m and Cmca) of IrB4 are predicted. The P63/mmc phase is the ground-state structure under ambient conditions, while C2/m and Cmca phases form at high pressure. The phase stabilities, mechanical and electronic properties of the three new phases of IrB4 are investigated systematically. We firstly found that pressure stimulates IrB4 to undergo phase transition twice, P63/mmc phase to C2/m phase at 29 GPa and C2/m phase to Cmca at 99 GPa. Strikingly, the three phases are both dynamically and mechanically stable under ambient conditions. The high bulk and shear moduli and low Poisson's ratio for the three phases in IrB4 make IrB4 a promising low-compressible material. Detailed analysis of density of states and electronic local functions reveals that the covalent bonding of Ir-B and B-B is responsible for their structural stability and high hardness. Finally, the high-pressure and high-temperature phase diagram of IrB4 is established, where the triple point of phase transition for P63/mmc-, C2/m- and Cmca-IrB4 is at 60 GPa and 1930 K.