Theoretical prediction of phase transition, mechanical and electronic properties of manganese diboride under pressure

The structure prediction algorithm CALYPSO combined with density functional theory calculations are used to find the high-pressure stable structure of manganese diboride (MnB₂) in the pressure range of 0–400 GPa. It is found that the pressure-induced phase transition sequence of MnB₂ should be P6₃/mmc (hP6-MnB₂) → Immm (oI18-MnB₂) → Pnma (oP12-MnB₂) → Imma (oI12-MnB₂), and the transition pressure is 35.9, 62.4, and 137.3 GPa, respectively. Among them, oP12-MnB₂ is a new stable orthorhombic phase in MnB₂ and never have been reported before. At ambient conditions, the calculated hardness of hP6-MnB₂ and oI18-MnB₂ is 34.5 and 22.9 GPa, respectively, suggesting that they are potentially hard material. However, oP12-MnB₂ and oI12-MnB₂ just possess a smaller hardness value of 13.9 and 16.2 GPa at the pressure of 80 and 140 GPa, respectively. In addition, the electronic structure and chemical bonds studies showed that the strong covalent bond formed by B-B bonds serve as the primary factor for maintain the structural stability of oP12-MnB₂ and oI12-MnB₂ under high pressure. Furthermore, at the pressure of 140 GPa, the B-B bonds of oP12 phase exists obvious antibonding characteristics below Femi level. The filling of anti-bonding states is detrimental to the stability of the crystal structure, which is the reason why the oP12 phase is less stable than the oI12 phase at this pressure.