First-principles study of multifunctional Mn₂B₃ materials with high hardness and ferromagnetism

Transition metal boride TM₂B₃ is widely studied in the field of physics and materials science. However, Mn₂B₃ has not been found in Mn-B systems so far. Mn₂B₃ undergoes phase transitions from <i>Cmcm</i> (0-28 GPa) to <i>C</i>2/<i>m</i> (28-80 GPa) and finally to <i>C</i>2/<i>c</i> (80-200 GPa) under pressure. Among these stable phases, <i>Cmcm</i>- and <i>C</i>2/<i>m</i>-Mn₂B₃s comprise six-membered boron rings and <i>C</i>2/<i>c</i>-Mn₂B₃ 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 TM₂B₃s 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 + <i>U</i> (<i>U</i> = 3.9 eV, <i>J</i> = 1 eV).