Proton Diffusion in Orthorhombic Perovskite Sulfides

The proton mobility in perovskite sulfides is investigated. Both stable as well as unstable compounds are considered to cover a wide range of ABS₃ compounds, the latter were selected based on a preferably small energy difference to the thermodynamic phase equilibrium. Density functional theory (DFT) is used to analyze all possible metastable hydrogen positions within the (001) and (110)/(11̅0) planes spanned by the sulfur atoms. The nudged elastic band (NEB) method is used to determine the activation energy barriers between neighboring hydrogen sites. From the hydrogen positions and the activation energies, the diffusion rate is calculated with an approach based on the Markovian master equation. Proton mobility is analyzed in detail for a subset of compounds, while a simplified analysis of the zigzag-paths in the prominent [001] and [010] directions is used to explore a wider chemical space. Room temperature diffusion coefficients of the order of 10^-6 cm²/s are predicted to be feasible in Zr-based compounds. The A- and B-site occupants influence mobility mainly due to their impact on crystallography, because symmetry-breaking distortions that reduce the S-S distance have a leading influence on reducing activation energies, but they also induce significant anisotropy.