Novel magnesium borides and their superconductivity

Abstract

With the motivation of searching for new superconductors in the Mg–B system, we performed ab initio evolutionary searches for all the stable compounds in this binary system in the pressure range of 0–200 GPa. We found previously unknown, yet thermodynamically stable, compositions MgB₃ and Mg₃B₁₀. Experimentally known MgB₂ is stable in the entire pressure range 0–200 GPa, while MgB₇ and MgB₁₂ are stable at pressures below 90 GPa and 35 GPa, respectively. We predict a reentrant behavior for MgB₄, which becomes unstable against decomposition into MgB₂ and MgB₇ at 4 GPa and then becomes stable above 61 GPa. We find ubiquity of phases with boron sandwich structures analogous to the AlB₂-type structure. However, with the exception of MgB₂, all other magnesium borides have low electron–phonon coupling constants λ of 0.32–0.39 and are predicted to have T_c below 3 K.