Phase transitions, mechanical properties and electronic structures of novel boron phases under high-pressure: A first-principles study

We have explored the mechanical properties, electronic structures and phase transition behaviors of three designed new phases for element boron from ambient condition to high-pressure of 120 GPa including (1) a C2/c symmetric structure (m-B₁₆); (2) a symmetric structure (c-B₅₆) and (3) a Pmna symmetric structure (o-B₂₄ ). The calculation of the elastic constants and phonon dispersions shows that the phases are of mechanical and dynamic stability. The m-B₁₆ phase is found to transform into another new phase (the o-B₁₆ phase) when pressure exceeds 68 GPa. This might offer a new synthesis strategy for o-B₁₆ from the metastable m-B₁₆ at low temperature under high pressure, bypassing the thermodynamically stable γ-B₂₈. The enthalpies of the c-B₅₆ and o-B₂₄ phases are observed to increase with pressure. The hardness of m-B₁₆ and o-B₁₆ is calculated to be about 56 GPa and 61 GPa, approaching to the highest value of 61 GPa recorded for α-Ga-B among all available Boron phases. The electronic structures and bonding characters are analyzed according to the difference charge-density and crystal orbital Hamilton population (COHP), revealing the metallic nature of the three phases.