First-principles Study of Multifunctional MnB Materials with High Hardness and Ferromagnetism

Transition metal boride TMB is widely studied in the field of physics and materials science. However, MnB has not been found in Mn-B systems so far. MnB undergoes phase transitions from (0-28 GPa) to 2/ (28-80 GPa) and finally to 2/ (80-200 GPa) under pressure. Among these stable phases, - and 2/-MnBs comprise six-membered boron rings and 2/-MnB has wavy boron chains. They all have good mechanical properties and can become potential multifunctional materials. The strong B-B covalent bonding is mainly responsible for the structural stability and hardness. Comparison of the hardness of the five TMBs with different bonding strengths of TM-B and B-B bonds reveals a nonlinear change in the hardness. According to the Stoner model, these structures possess ferromagnetism, and the corresponding magnetic moments are almost the same as those of GGA and GGA + ( = 3.9 eV, = 1 eV).