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

Abstract

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.