Prediction of high- Tc superconductivity in ternary lanthanum borohydrides

The study of superconductivity in compressed hydrides is of great interest due to measurements of high critical temperatures $({T}_{c})$ in the vicinity of room temperature, beginning with the observations of ${\mathrm{LaH}}_{10}$ at 170--190 GPa. However, the pressures required for synthesis of these high-${T}_{c}$ superconducting hydrides currently remain extremely high. Here we show the investigation of crystal structures and superconductivity in the La-B-H system under pressure with particle-swarm intelligence structure-searches methods in combination with first-principles calculations. Structures with seven stoichiometries, LaBH, ${\mathrm{LaBH}}_{4}, {\mathrm{LaBH}}_{6}, {\mathrm{LaBH}}_{7}, {\mathrm{LaBH}}_{8}$, La(${\mathrm{BH})}_{3}$, and $\mathrm{La}{({\mathrm{BH}}_{4})}_{3}$ were predicted to become stable under pressure. Remarkably, the hydrogen atoms in ${\mathrm{LaBH}}_{8}$ were found to bond with B atoms in a manner that is similar to that in ${\mathrm{H}}_{3}\mathrm{S}$. Lattice dynamics calculations indicate that ${\mathrm{LaBH}}_{7}$ and ${\mathrm{LaBH}}_{8}$ become dynamically stable at pressures as low as 109 and 48 GPa, respectively. Moreover, the two phases were predicted to be superconducting with a critical temperature ${T}_{c}$ of 93 K and 156 K at 110 GPa and 55 GPa, respectively (${\ensuremath{\mu}}^{*}$ = 0.1). The present results provide guidance for future experiments targeting hydride superconductors with both low synthesis pressures and high ${T}_{c}$.