Complex phase transition and asymmetric ferroelectricity of homochiral secondary ammonium salts

Chirality, an inherent asymmetry in nature, continues to captivate scientists due to its pivotal role in material assembly and the resulting properties. In particular, homochiral molecules can be introduced to offer a significant likelihood of forming crystals with polar point groups, enabling the manifestation of their ferroelectricity. With reference to the light-weight, environmentally friendly, and ferroelectric secondary ammonium salts (SASs), data mining of the existing crystal database was performed to locate three rotator-stator-type SASs with asymmetric ferroelectricity. Noticeably, these SASs contain homochiral organic cations. Then, reasonable chemical modification was further implemented to design five new SASs still with asymmetric ferroelectricity. The first-principles calculations were employed to demonstrate that these designed SASs undergo a ferroelectric-paraelectric-ferroelectric phase transition triggered by the rotation of the secondary ammonium ions and accompanied by the breakage and formation of the armchair NH⋯Cl hydrogen bonds and displacement of Cl^- ions. The specific components, secondary ammonium ions, and assembled crystal structures of new SASs bring an asymmetric chemical environment for their phase transitions and finally lead to their asymmetric ferroelectricity. Our study provides a design strategy to tailor homochiral molecular crystals with asymmetric ferroelectricity and identifies a class of potential ferroelectric candidates for next generation intelligent electronics.