Pressure-Induced Anion Order-Disorder Transition in Layered Perovskite Sr 2 LiHOCl 2 .

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 Sr 2 CuO 2 Cl 2 -type layered perovskite Sr 2 LiHOCl 2 exhibits a pressure-induced H - /O 2- order-disorder transition. When synthesized at ambient and low pressures (≤2 GPa), Sr 2 LiHOCl 2 is isostructural to orthorhombic Eu 2 LiHOCl 2 ( 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 ( I 4/ mmm ) with a loss of the superstructure. The structural analysis revealed that, in the ambient pressure phase, HLi 2 Sr 4 and OLi 2 Sr 4 octahedra have distinct sizes to stabilize otherwise underbonded oxide ions, which is less important at the higher pressure. Anion-disordered Sr 2 LiHOBr 2 and Ba 2 LiHOCl 2 were also obtained at 5 GPa. Given the abundant layer-type anion order in perovskite-based oxyhydrides (e.g., La 2 LiHO 3 ), 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.