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 MgB and MgB. Experimentally known MgB is stable in the entire pressure range 0-200 GPa, while MgB and MgB are stable at pressures below 90 GPa and 35 GPa, respectively. We predict a reentrant behavior for MgB, which becomes unstable against decomposition into MgB and MgB at 4 GPa and then becomes stable above 61 GPa. We find ubiquity of phases with boron sandwich structures analogous to the AlB-type structure. However, with the exception of MgB, all other magnesium borides have low electron-phonon coupling constants λ of 0.32-0.39 and are predicted to have T below 3 K.