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 FA(2)PbBr(4) composition and investigated the prerequisites of their formation on the crystallization of FABr-excessive solutions of FAPbBr(3). 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 FA(2)PbBr(4) phases usually exist in a labile equilibrium with the FAPbBr(3) 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 FA(2)PbBr(4) and to corroborate the conclusions on their optoelectronic properties. As a result, we found that FA(2)PbBr(4) phases irradiated by UV can exhibit effective green photoluminescence due to the transfer of excitation energy to defective states or 3D perovskite inclusions.

Automated summary

Two novel low-dimensional phases with FA(2)PbBr(4) composition were discovered in this study, showing labile equilibrium with the FAPbBr(3) three-dimensional perovskite and potential to form composites. The optical properties of both polymorphs were comprehensively studied using absorption spectroscopy, diffuse reflection spectroscopy, and photoluminescence spectroscopy. The FA(2)PbBr(4) phases were found to exhibit effective green photoluminescence when irradiated by UV, possibly due to energy transfer to defective states or 3D perovskite inclusions.