Fluorine Substitution in Ethylamine Triggers Second Harmonic Generation in Noncentrosymmetric Crystalline [NH₃CH₂CH₂F]₃BiCl₆

The isotope effect usually brings improved performance or new features, such as KD2PO4 and SrTi18O3 inducing a higher phase transition temperature (Tc) and ferroelectricity, respectively. However, the isotope effect seems to work in some limited systems and is usually extremely toxic and unstable. Due to the close van der Waal’s radii and the similar steric parameters, the H/F substitution as an isotope-like effect has been used on spherical amines for designing ferroelectrics. In this work, we reported a study of H/F substitution based on the chain amine in [NH3CH2CH3]3BiCl6. As expected, the substituted compound [NH3CH2CH2F]3BiCl6 successfully achieves the space group transformation from a centrosymmetric to a noncentrosymmetric system, triggering the second-order nonlinear optical properties. Equally importantly, the phase transition temperature increases significantly from 229 to 361.5 K. The crystal structure analysis and 2D fingerprint plots prove that the excellent properties stemmed from the increased force between the organic cations and the inorganic parts as temperature increases. Consequently, this work will open the door for designing a chain amine in hybrid materials to improve the original performance or generate a new performance.