Oxidation-induced phase transformations of hybrid tin bromide single crystals enable the occurrence of second-harmonic generation

Phase transformations of tin halide hybrid materials play an important role in the formation of structural diversity and optoelectronic versatility due to the oxidability and instability from Sn²⁺ to Sn⁴⁺. Here, we demonstrate the synthesis, growth, phase transformations and fundamental properties of DMESnBr_m (DME²⁺ = N,N′-dimethylethylenediaminium; m = 4 or 6) and TMDPSn_nBr₆ (TMDP²⁺ = 4,4′-trimethylenedipyridinium; n = 1 or 2) single crystals. Interestingly, both DMESnBr₄ and TMDPSn₂Br₆ single crystals undergo irreversible dissolution-recrystallization-induced phase transformations from Sn²⁺ to Sn⁴⁺ in the HBr-H₃PO₂ mixed solution when exposed to ambient atmosphere, which easily results in the formation of DMESnBr₆ and TMDPSnBr₆. In particular, DMESnBr₆ crystallizes in the non-centrosymmetric space group P2₁ (no. 4) by single-crystal X-ray diffraction and piezoelectric measurements. The second harmonic generation (SHG) response of DMESnBr₆ is about 0.5 times that of potassium dihydrogen phosphate (KDP) with type-I phase matching behaviors. Structural and theoretical analyses show that the SHG effect is attributed to the asymmetrical displacement of {SnBr₆} octahedra. The oxidation-induced phase transformation behaviors provide new insights into the design and acquisition of novel Sn-based hybrid optoelectronic materials.