  Small cations such as guanidinium and cesium can act as templating cations to
form low dimensional phases (2D, 1D, 0D) in the case of excess of organic
halides. However, such phases with the widely used formamidinium (FA+) cation
have not been reported so far. In this study, we discovered two novel low
dimensional phases with a composition of FA2PbBr4 and investigated the
prerequisites of their formation upon 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 FAPbBr3 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. 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 a transfer of excitation energy to defective states or
3D perovskite inclusions.
