Pressure-Induced Anion Order-Disorder Transition in Layered Perovskite Sr₂LiHOCl₂

While cation order-disorder transitions have been extensively investigated because of their decisive impact on chemical and physical properties, only few anion order-disorder transitions are known. Here, we show that Sr₂CuO₂Cl₂-type layered perovskite Sr₂LiHOCl₂ exhibits a pressure-induced H^-/O^2- order-disorder transition. When synthesized at ambient and low pressures (≤2 GPa), Sr₂LiHOCl₂ is isostructural to orthorhombic Eu₂LiHOCl₂ (Cmcm) with a H^-/O^2- order at the equatorial sites. However, applying a higher pressure (5 GPa) during synthesis causes the equatorial anions to be disordered, leading to a tetragonal symmetry (I4/mmm) with a loss of the superstructure. The structural analysis revealed that, in the ambient pressure phase, HLi₂Sr₄ and OLi₂Sr₄ octahedra have distinct sizes to stabilize otherwise underbonded oxide ions, which is less important at the higher pressure. Anion-disordered Sr₂LiHOBr₂ and Ba₂LiHOCl₂ were also obtained at 5 GPa. Given the abundant layer-type anion order in perovskite-based oxyhydrides (e.g., La₂LiHO₃), the inclusion of additional anions (e.g., chloride) expands the frontiers of anion ordering patterns and their distribution control with the benefit of improving ionic conduction in solids.