Broad-Band Emission Switch in Room Temperature Stable Hybrid Perovskite Polymorphs

Low dimensional halide perovskites exhibiting broad band emission have attracted increasing attention owing to their potentiality in lighting applications. However, the relationships between the structure and luminescence properties remain unclear up to now. Here, we report a bromoplumbate organic-inorganic perovskite (C3aH)(2)PbBr4 (C3aH(+): 3-ammonium propionic acid cation) which exists as two polymorphs at room temperature (RT): the thermodynamically stable alpha-phase crystallizing in the P2(1)/n monoclinic space group and the metastable beta-phase crystallizing in the P2(1)/c monoclinic space group. alpha-(C3aH)(2)PbBr4 can be transformed into beta-(C3aH)(2)PbBr4 by heating over 120 degrees C, while the reversible transformation occurs upon cooling at very low temperature (100 K) or upon a water or acetone atmosphere. Both polymorphs are built from < 100 > layered perovskites separated by the same C3aH(+) cations. No out-of-plane distortion of perovskite layers is observed in both structures; but strong in-plane distortions with regular PbBr6 octahedra are observed in the beta-phase, whereas the alpha-phase exhibits strongly distorted PbBr6 revealing the 6s(2) lone pair stereoactivity. Interestingly, beta-(C3aH)(2)PbBr4 exhibits a narrow emission band (blue emission), whereas alpha-(C3aH)(2)PbBr4 exhibits both narrow and broad bands (whitish emission). The existence of two perovskite polymorphs with different emission properties at a given temperature not only allows the establishment of structure-property relationships but also demonstrates that alpha-(C3aH)(2)PbBr4 can act as a room temperature luminescence switchable material.