Understanding ionic transport in perovskite lithium-ion conductor Li_3/8Sr_7/16Ta_3/4Hf_1/4O₃: a neutron diffraction and molecular dynamics simulation study

Solid-state Li-ion electrolytes (SSEs) are essential for the development of next-generation solid-state Li-metal batteries and new Li-extraction electrochemical cells. Among these, the perovskite-type SSE Li₃/₈Sr₇/₁₆Ta₃/₄Hf₁/₄O₃ (LSTH) has garnered attention for Li-extraction applications, owing to its outstanding chemical and thermal stability and high ionic conductivity. However, its precise crystal structure and Li-ion transport mechanisms remain insufficiently understood. This study addresses these gaps by employing neutron diffraction to resolve LSTH's crystallography and machine learning force field (MLFF) based MD simulations to elucidate ionic transport mechanisms. A single-phase LSTH, synthesized via the sol-gel method, exhibits a room-temperature bulk conductivity of 0.418 mS cm⁻¹ and a relative density of 98%. Neutron diffraction reveals 2.625 Li vacancies per unit cell, with the remaining 0.375 Li occupying the unconventional Wyckoff position (24k), different from the traditional A-site position (1a) of regular perovskites. This unique crystallography suggests a “zig-zag” Li-ion migration pathway via vacancies. However, MLFF based MD simulations suggest that Li ions at (24k), compared to (1a) occupancy, have limited mobility due to strong Li-vacancy ordering at low temperatures, leading to higher activation energy barriers and lower ionic conductivity. These findings underscore the critical influence of Li-site occupancy on ionic conductivity and provide structural insights for designing high-conductivity SSEs.