Geometric frustration unlocks quantum delocalization: The mechanism of cryogenic transport in LiTi2(PS4)3

The geometrically frustrated superionic conductor LiTi 2 (PS 4 ) 3 (LTPS) exhibits exceptionally fast Li exchange at cryogenic temperatures, with an apparent activation energy as low as ∼39 meV and persistent low-temperature NMR line broadening suggesting the involvement of nuclear quantum effects (NQEs). Here, we investigate the microscopic origin of this behavior using thermostatted ring-polymer molecular dynamics (TRPMD) driven by a high-fidelity neuroevolution potential (NEP), enabling quantum-statistical sampling of the highly anharmonic potential-energy surface. Starting from an independently calculated static migration barrier of 65.2 meV, we show that zero-point energy corrections reduce the adiabatic barrier to 54.9 meV, while full-dimensional TRPMD sampling further renormalizes the free-energy barrier to 42.6 meV. This reduction arises from the synergy of two distinct quantum contributions: a static zero-point energy lift associated with the confinement of Li in the resting state and a dynamic phonon-mediated wavepacket-shaping mechanism. Near the transition state, transient breathing of the sulfur bottleneck is coupled to pronounced directional elongation of the Li nuclear wavepacket, thereby reducing steric hindrance and facilitating barrier crossing. Simulated lattice expansion further demonstrates that relieving geometric frustration deepens the classical potential trap and suppresses the quantum-assisted transport advantage. These results provide a quantitative microscopic basis for the experimentally proposed involvement of NQEs in cryogenic Li dynamics and identify geometric frustration as a key structural condition for enhancing quantum-assisted ion transport in crystalline solid electrolytes.