Tunable phase transition, band gap and SHG properties by halogen replacement of hybrid perovskites [(thiomorpholinium)PbX3, X = Cl, Br, I]

By the replacement of halogen anion, three new multifunctional organic-inorganic hybrid perovskites (thiomorpholinium)PbX3 (X = Cl, Br, I) were successfully synthesized and underwent reversible structural transformation above room temperature, accompanied by the anomalous change of dielectric constant. With the adjustment of the halogen anion from Cl to I in the inorganic skeleton, the space group is transformed from centrosymmetric space group P21/c ((thiomorpholinium)PbCl3) to chiral one P212121 ((thiomorpholinium)PbBr3, (thiomorpholinium)PbI3) at room temperature. The ordered-disordered transition of organic cations and the change of hydrogen bonds with the increase of temperature lead to above-room-temperature phase transitions. Ultraviolet absorption and second-harmonic generation (SHG) measurements confirmed that both the band gap and SHG activity of (thiomorpholinium)PbX3 (X = Cl, Br, I) crystals were tunable. The band gaps reveal a broadening trend with 3.532 eV, 3.410 eV and 3.175 eV along the Cl → Br → I series. This work provides an effective molecular design for multifunctional organic-inorganic perovskites.