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.