The pressure-induced phase transition and mechanical properties of rhenium diboride (ReB2) have been investigated by using the particle swarm optimization algorithm in combination with density functional theory calculations. It was found that the P63/mmc structure of ReB2 (hP6-ReB2) is the most stable phase at ambient conditions, and it will transform to P6/mmm structure (hP3-ReB2) at about 300.7 GPa. Phonon dispersion curve calculations suggest that hP6-ReB2 and hP3-ReB2 possess dynamical stabilities in a wide range of 0–400 and 120–400 GPa, respectively. Moreover, the calculated hardness of hP6-ReB2 is 38.1 GPa at ambient conditions, suggesting it is a potential hard material. However, hP3-ReB2 just possesses the hardness value of 18.9 GPa at the pressure of 120 GPa. Finally, by means of the quasiharmonic approximation method, the high-pressure and finite-temperature phase diagram of ReB2 is proposed. It is found that the transition pressure of hP6-ReB2 to hP3-ReB2 decreases with increasing temperatures.
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.
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