Prediction of High-Tc conventional Superconductivity in the Ternary Lithium Borohydride System

We investigate the superconducting ternary lithium borohydride phase diagram at pressures of 0 and 200$\,$GPa using methods for evolutionary crystal structure prediction and linear-response calculations for the electron-phonon coupling. Our calculations show that the ground state phase at ambient pressure, LiBH$_4$, stays in the $Pnma$ space group and remains a wide band-gap insulator at all pressures investigated. Other phases along the 1:1:$x$ Li:B:H line are also insulating. However, a full search of the ternary phase diagram at 200$\,$GPa revealed a metallic Li$_2$BH$_6$ phase, which is thermodynamically stable down to 100$\,$GPa. This {\em superhydride} phase, crystallizing in a $Fm\bar{3}m$ space group, is characterized by six-fold hydrogen-coordinated boron atoms occupying the $fcc$ sites of the unit cell. Due to strong hydrogen-boron bonding this phase displays a critical temperature of $\sim$ 100$\,$K between 100 and 200$\,$GPa. Our investigations confirm that ternary compounds used in hydrogen-storage applications are a suitable choice for observing high-$T_\text{c}$ conventional superconductivity in diamond anvil cell experiments, and suggest a viable route to optimize the critical temperature of high-pressure hydrides.