Hydrogen defects in LaBi2O4X (X = Cl, Br, and I) Sillén oxyhalide phases and their impacts on ionic transport

Abstract

Sillén oxyhalides have recently emerged as promising materials for both photocatalytic and ionic transport applications, yet the role of likely-ubiquitous hydrogen-related defects in these layered compounds remains largely unexplored. Here, we employ first-principles defect calculations to investigate incorporation energetics for hydrogen- and oxygen-related defects, as well as their migration barriers in LaBi₂O₄X (X = Cl, Br, I) phases. We find that hydrogen interstitials, particularly protonic species (H_i⁺), are readily accommodated within the open Bi–O layers. Protons compete with oxygen vacancy donors (V_O²⁺) and charge-compensate with oxygen interstitial acceptors (O_i²⁻). By linking hydrogen defect formation to water- and oxygen-related redox equilibria, we reveal that V_O²⁺ facilitates H_i⁺ incorporation, while O_i²⁻ promotes interstitial hydroxide formation, establishing a direct connection between proton and oxide-ion transport. Calculated migration barriers indicate that ionic diffusion is confined to Bi–O layers with low barriers of 0.20–0.25 eV for H_i⁺ and 0.14–0.25 eV for V_O²⁺, suggesting that the materials contain intrinsic pathways for mixed ionic conduction. These results provide a microscopic picture of hydrogen behavior in Sillén oxyhalides and point to design strategies for integrating protonic and oxide-ion transport in layered oxyhalide electrolytes. Band-edge alignment analysis shows that LaBi₂O₄I provides the optimal combination of hydrogen solubility, oxygen defect stability, and mixed ionic conductivity, highlighting its potential for low-temperature electrochemical and energy-conversion applications. Overall, this work establishes the defect-driven origin of hydrogen transport in Sillén oxyhalides and expands their applicability beyond photocatalysis to mixed ionic conduction and hydrogen electrochemistry.