First-principles study of multifunctional Mn 2 B 3 materials with high hardness and ferromagnetism.

Transition metal boride TM 2 B 3 is widely studied in the field of physics and materials science. However, Mn 2 B 3 has not been found in Mn-B systems so far. Mn 2 B 3 undergoes phase transitions from Cmcm (0-28 GPa) to C 2/ m (28-80 GPa) and finally to C 2/ c (80-200 GPa) under pressure. Among these stable phases, Cmcm - and C 2/ m -Mn 2 B 3 s comprise six-membered boron rings and C 2/ c -Mn 2 B 3 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 TM 2 B 3 s 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 + U ( U = 3.9 eV, J = 1 eV).