Impact of ionic quantum fluctuations on the thermodynamic stability and superconductivity of LaBH8

The recent prediction of a metastable high-symmetry $Fm\overline{3}m$ phase of ${\mathrm{LaBH}}_{8}$ gives hopes to reach high superconducting critical temperatures at affordable pressures among ternary hydrogen-rich compounds. Making use of first-principles calculations within density functional theory and the stochastic self-consistent harmonic approximation, we determine that ionic quantum fluctuations drive the system dynamically unstable below 77 GPa, a much higher pressure than the 45 GPa expected classically. Quantum anharmonic effects stretch the covalent B-H bond in the ${\mathrm{BH}}_{8}$ units of the structure, and consequently, soften all hydrogen-character modes. Above 77 GPa $Fm\overline{3}m\phantom{\rule{4pt}{0ex}}{\mathrm{LaBH}}_{8}$ remains metastable, and interestingly, its superconducting critical temperature is largely enhanced by quantum anharmonic effects, reaching critical temperatures around 160 K at the verge of the dynamical instability. Our results suggest that low-pressure metastable phases with covalently bonded symmetric ${\mathrm{XH}}_{8}$ units will be destabilized by ionic quantum fluctuations.