New Low-Dimensional Lead-Free Perovskite (2-AMP)₂BiX₇·H₂O (X = Cl, Br) Crystals: Synthesis, Stability, and Nonlinear...

Organic-inorganic hybrid Pb-halide perovskites have attracted tremendous interest due to their potential application in photovoltaic and optoelectronic fields. However, the toxicity and poor stability of Pb-halide perovskites have become key bottlenecks toward their future commercialization and industrialization. Therefore, in this work, two novel hybrid lead-free perovskite nonlinear optical (NLO) crystals (2-AMP)₂BiX₇·H₂O (X = Cl, Br) with high stability were successfully synthesized. Both the crystals belong to the orthorhombic P2₁2₁2₁ space group, displaying a zero-dimensional perovskite structure. The thermal, environmental, and solvent atmospheric stabilities were comprehensively investigated, with high thermal stability up to 170 °C and high environmental and high solvent atmospheric stability observed for (2-AMP)₂BiBr₇·H₂O. First-principles calculations were carried out to study the relationship of the structure and properties. A large birefringence of 0.1368@1064 nm was determined for (2-AMP)₂BiBr₇·H₂O, which was derived from the strong aeolotropic conjugated π-electron distribution of planar 2-aminomethylpyridium. A second-harmonic generation (SHG) effect that was 0.25 and 0.32 times that of KH₂PO₄ (KDP) was measured for (2-AMP)₂BiCl₇·H₂O and (2-AMP)₂BiBr₇·H₂O, respectively, and the stronger SHG response of bromide was attributed to the larger distortions of {BiBr₆} octahedrons. This work may offer new guidance for exploration of low toxicity and high stability of perovskite NLO materials.