FA₂PbBr₄: Synthesis, Structure, and Unusual Optical Properties of Two Polymorphs of Formamidinium-Based Layered (110) Hybrid Perovskite

Small cations such as guanidinium and cesium can act as templating cations to form low-dimensional perovskite-like phases (two-dimensional (2D), one-dimensional (1D), zero-dimensional (0D)) in the case of an excess of organic halides. However, such phases with the widely used formamidinium (FA+) cations have not been reported so far. In this study, we discovered two novel low-dimensional phases with FA2PbBr4 composition and investigated the prerequisites of their formation on the crystallization of FABr-excessive solutions of FAPbBr3. We found that both phases have the structure of (110) layered perovskite but are represented by two different polymorphs with "eclipsed"and "staggered"arrangement of adjacent layers. It was shown that FA2PbBr4 phases usually exist in a labile equilibrium with the FAPbBr3 three-dimensional (3D) perovskite and can form composites with it. The optical properties of both polymorphs were comprehensively studied by means of absorption spectroscopy, diffuse reflection spectroscopy, and photoluminescence spectroscopy. Density functional theory (DFT) calculations were applied to investigate the band structure of the FA2PbBr4 and to corroborate the conclusions on their optoelectronic properties. As a result, we found that FA2PbBr4 phases irradiated by UV can exhibit effective green photoluminescence due to the transfer of excitation energy to defective states or 3D perovskite inclusions.