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 DMESnBrm (DME2+ = N,N′-dimethylethylenediaminium; m = 4 or 6) and TMDPSnnBr6 (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.