3D Perovskite (1,5-3.2.2-H 2 dabcn)CsBr 3 with Reverse Symmetry Breaking.

Even though hybrid organic-inorganic perovskites (HOIPs) have been studied by many scholars in recent years, there are not many reports on three-dimensional (3D) alkali metal cesium halide perovskites. Here, we report an unprecedented 3D HOIP molecule (1,5-3.2.2-H 2 dabcn)CsBr 3 ( 1 ), in which the 3D anionic framework is constructed by corner-sharing CsBr 6 octahedra and organic cations [1,5-3.2.2-H 2 dabcn] 2+ are located in the cavities formed by the octahedra. Organic cations interact with an inorganic metal frame via two N-H···Br hydrogen bonds. Compound 1 undergoes a reversible order-disorder phase transition and exhibits switchable dielectric and second-harmonic generation (SHG) properties. Interestingly, product 1 crystallizes in a non-centrosymmetric space group Pmn 2 1 at the low-temperature phase (LTP) and transforms into a centrosymmetric space group P 2/ m at the high-temperature phase (HTP). The space group Pmn 2 1 in the LTP has a higher symmetry than P 2/ m in the HTP. This inverted symmetry breaking is very unusual.