Impact of Lattice Heterogeneity on Broadband Optical Retardation in Stacked 2D Perovskite Single Crystals

Abstract 2D metal‐halide perovskites (2D‐MHPs) are renowned for their electronic and optical applications, thanks to their distinctive layered structures comprising nanometer‐thick stacked metal‐halide layers intercalated with organic cations. This structural arrangement, however, also introduces a level of disorder that significantly influences the physical and electronic properties of these materials. The impact of organic layers on long‐range disorder, particularly crystal mosaicity in 2D‐MHPs, remains poorly understood. Here, the impact of interfaces is reported between organic and inorganic layers on intrinsic crystal structural disorder and crystal mosaicity in 2D‐MHPs single crystals. 2D‐MHPs with single and mixed organic cations are synthesized, where single‐cation crystals exhibit AB‐AB stacking while ordered mixed‐cation compounds exhibit ABCD‐ABCD stacking within the unit cell. Single crystal X‐ray diffraction (SCXRD) analysis revealed that mixed cation 2D‐MHPs showed significant long‐range structural disorder. Moreover, the high in‐plane birefringence values of mixed cation 2D‐MHPs indicate the in‐plane anisotropic change. Due to substantial crystal mosaicity and in‐plane birefringence, the mixed cation bulk crystal showed a broadband optical retardation, measured by transmission optical modulation spectroscopy (TOMS). This research not only deepens understanding of structural disorder in 2D‐MHPs but also fosters the development of materials engineered for specific optical functionalities.