Hydrogen defect stability and dynamics in Sillen Oxyhalids

Sillén oxyhalides, featuring alternating fluorite-like [M₃O₄]⁺ slabs and halide layers, exhibit tunable electronic and optical properties, positioning them as attractive candidates for photocatalysis. Recent discoveries of high oxygen-ion conductivity in LaBi₂O₄Cl and its famly further underscore their potential in energy-related applications, including fuel cells, electrolyzers, and solid-state batteries.

In this work, we explore the role of hydrogen-related defects in Sillén phases using density functional theory (DFT). Focusing on LaBi₂O₄X (X=Cl, Br, I), we evaluate the formation energetics and migration kinetics of hydrogen and oxygen defects. Our calculations reveal that H_i⁺ and V_O²⁺ are the most energetically favorable defects, coexisting within the band gap. By referencing band edges to the vacuum level, we determine the electrochemical conditions under which hydrogen incorporation becomes dominant and find that LaBi₂O₄I exhibits the highest H_i⁺ solubility among the compounds considered. Migration barriers of H_i⁺ (0.20–0.25 eV) are found to be comparable to those for V_O²⁺ (0.14–0.25 eV), indicating the potential for concurrent proton and oxygen transport. Notably, LaBi₂O₄I shows the lowest (highest) migration barrier for H_i⁺ (V_O²⁺), whereas LaBi₂O₄Cl displays the opposite behavior. This study highlights the promise of Sillén oxyhalides as mixed ionic conductors, extending their relevance beyond photocatalysis toward broader electrochemical energy applications.