Mixed Bromine–Chlorine Induced Great Dielectric and Second-Order Nonlinear Optical Properties Changes in Phase Transitions Compounds [H2mdap][BiBr5(1‑x)Cl5x] (x = 0.00–1.00)

Doping/mixing method plays an important role in modulating the structures and physical properties of functional materials. Herein, we report that the mixed crystals [H2mdap]­[BiBr5(1‑x)Cl5x] (H2mdap = protonated N-methyl-1,3-diaminopropane, x = 0.00–1.00) undergo phase transitions with the trend that the larger x, the higher phase transition temperature. Variable-temperature structure analysis shows that anionic and cationic moieties in the crystal lattice contribute to the phase transition. Room-temperature structure comparisons reveal that phase transition point moving to a higher temperature is mainly due to the decrease of the Bi–Cl/Br–Bi angles in the anionic chains and slightly alter the intermolecular interactions with mixed different components of halogen atoms Cl and Br. Dielectric and second harmonic generation experiments demonstrate their similar phase transition behaviors and coexistence of switchable dielectric behaviors and bistable nonlinear optical effects. Mixed Br–Cl induced great dielectric and second-order nonlinear optical properties changes which makes [H2mdap]­[BiBr5(1‑x)Cl5x] (x = 0.00–1.00) become potential tunable and multifunctional optical–electrical coupled materials.掺杂/混晶法对功能材料的结构与物理性能调控具有重要意义。本文报道了混晶[H₂mdap][BiBr₅(₁₋ₓ)Cl₅ₓ]（H₂mdap即质子化N-甲基-1,3-二氨基丙烷，x取值范围为0.00~1.00）的相变行为，其规律为x值越大，相变温度越高。变温结构分析显示，晶格中的阴、阳离子基团共同参与了相变过程。室温结构对比分析表明，相变点向高温迁移的主要原因是阴离子链中Bi–Cl/Br–Bi键角的减小，而混合Cl与Br两种不同卤素组分也会轻微改变分子间相互作用。介电性能测试与二阶谐波生成（second harmonic generation）实验结果证实，该系列晶体展现出相似的相变行为，且可切换介电特性与双稳态非线性光学效应共存。Br与Cl的混合体系引发了显著的介电性能与二阶非线性光学性能变化，使得[H₂mdap][BiBr₅(₁₋ₓ)Cl₅ₓ]（x=0.00~1.00）成为具备应用潜力的可调谐多功能光-电耦合材料。