Nuclear quantum and H/D isotope effects on hydrogen-bond symmetrization in lithium hydroxide crystals at high pressure

Molecular crystals with hydrogen bonds undergo a phase transition and symmetrization of the hydrogen-bond network at high pressures. Lithium hydroxide (LiOH) also undergoes a similar phenomenon; however, the pressure at which it occurs remains unclear. This study employed the path integral molecular dynamics (PIMD) and static density functional theory calculations to investigate hydrogen-bond symmetrization in LiOH crystals at high pressures. The nuclear quantum effects centralized the protons and significantly lowered the pressure required for hydrogen-bond symmetrization (∼500 GPa in PIMD, compared with the 1200 GPa recorded in the static density functional theory calculations). Furthermore, the pressure required for symmetrization in the deuterated system [PIMD(D)] was higher than that required in the nondeuterated system [PIMD(H)]. This indicated that the H/D isotope effect significantly affects the hydrogen-bond symmetrization. These results demonstrate that nuclear quantum and isotope effects significantly affect hydrogen-bond symmetrization in LiOH under extreme conditions.