Localized Lattice Expansion of FAPbBr3 to Design a 3D Hybrid Perovskite for Sensitive Near‐Infrared Photodetection.

A mixed‐cation 3D lead bromide hybrid perovskite (NMDAP)2FAPb4Br13 (1, NMDAP2+ = N‐methyl‐1,3‐diaminopropanium, FA+ = formamidinium) is tailored by incorporating the large NMDAP2+ cation with the small FA+ ion into 3D FAPbBr3. Structurally, the small FA+ ions occupy the prototypical cavities formed by distorted corner‐sharing PbBr6 octahedra as in FAPbBr3, while the bulky NMDAP2+ ions are confined to the expanded lattice which is defined by a new structural motif composed of eight corner‐sharing PbBr6 octahedra and four edge‐sharing octahedra. This localized lattice expansion from FAPbBr3 creates a new branch of the intriguing 3D hybrid perovskite family, breaking the limits of the conventional Goldschmidt tolerance factor rule. Moreover, a high two‐photon absorption (2AP) coefficient of 60.8 cm MW−1 was demonstrated to near‐infrared (NIR) 800 nm illumination, enabling sensitive NIR photoresponses with large on/off ratio (≈104) as well as favorable stability. [ABSTRACT FROM AUTHOR]