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 MgB${}_3$ and Mg${}_3$B${}_{10}$. Experimentally known MgB${}_2$ is stable in the entire pressure range 0-200 GPa, while MgB${}_7$ and MgB${}_{12}$ are stable at pressures below 90 GPa and 35 GPa, respectively. We predict a reentrant behavior for MgB${}_4$, which becomes unstable against decomposition into MgB${}_2$ and MgB${}_7$ at 4 GPa and then becomes stable above 61 GPa. We find ubiquity of phases with boron sandwich structures analogous to the AlB${}_2$-type structure. However, with the exception of MgB${}_2$, all other magnesium borides have low electron-phonon coupling constants $\lambda$ of 0.32 to 0.39 and are predicted to have T${}_c$ below 3 K.

13 pages, 10 figures. This manuscript has Supplemental Materials

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