Crystal structures and mechanical properties of osmium diboride at high pressure

We have investigated the crystal structures and mechanical properties of osmium diboride (OsB₂) based on the density functional theory. The structures of OsB₂ from 0 to 400 GPa were predicted using the particle swarm optimization algorithm structure prediction technique. The orthorhombic Pmmn structure of OsB₂ (oP6-OsB₂) was found to be the most stable phase under zero pressure and it will transfer to the hexagonal P6₃/mmc structure (hP6-OsB₂) around 12.4 GPa. Meanwhile, we have discovered a new stable orthorhombic Immm structure (oI12-OsB₂) above 379.6 GPa. After that, a thorough and comprehensive investigation on mechanical properties of different OsB₂ phases is performed in this work. Further studies showed that the hardness of oP6-OsB₂ and hP6-OsB₂ at zero pressure is 15.6 and 20.1 GPa, while that for oI12-OsB₂ under 400 GPa is 15.4 GPa, indicating that these three phases should be potentially hard materials rather than superhard materials. Finally, the pressure-temperature phase diagram of OsB₂ is constructed for the first time by using the quasi-harmonic approximation method. Our results showed that the transition pressures of oP6-OsB₂ → hP6-OsB₂ and hP6-OsB₂ → oI12-OsB₂ all decreases appreciably with the increase of temperature.