Novel magnesium borides and their superconductivity†
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 Tc below 3 K.