Variable-composition structural optimization and experimental verification of MnB3 and MnB4

Abstract

In combination with variable-composition evolutionary algorithm calculations and first-principles calculations, we have systematically searched for all the stable compounds and their crystal structures in the extensively investigated binary Mn–B system. Our results have uncovered four viable ground-state compounds, with Mn₂B, MnB, and MnB₄, and previously never reported MnB₃ and two metastable compounds, MnB₂ and Mn₃B₄. Our calculations demonstrate that the early characterized mC10 structure of MnB₄ showed dynamic instability with large imaginary phonon frequencies and, instead, a new mP20 structure is predicted to be stable both dynamically and thermodynamically, with a considerable energy gain and no imaginary phonon frequencies. The new MnB₃ compound crystallizes in the monoclinic mC16 structure which lies 3.2 meV per atom below the MnB (oP8) ↔ MnB₄ (mP20) tie-line at T = 0 K. Furthermore, these proposed phases have been verified by our annealed samples after arc-melting synthesis and corresponding powder XRD measurements.