Spacer Cation Alloying of a Homoconformational Carboxylate trans Isomer to Boost in-Plane Ferroelectricity in a 2D Hybrid Perovskite

Two-dimensional (2D) hybrid perovskites of Ruddlesden-Popper (RP) lattices are recently booming as a vigorous class of ferroelectrics, whereas their intrinsic van der Waals gaps exert weak interactions that destabilize the layered motifs. Thus, it is an urgent challenge to reduce interlayered energy gaps to allow an exploration of stable RP ferroelectrics. Here, we propose hydrogen bonds to reduce van der Waals gaps of 2D RP-type perovskites while the ferroelectricity is retained. For the first time, a homoconformational trans isomer has been alloyed as the spacing cation of the 2D ferroelectric (t-ACH)(2) (EA)(2)Pb3Br10 (1, where t-ACH is 4-amino-methyl-1-cyclohexanecarboxylate and EA is ethylammonium). Strikingly, the strong O-H center dot center dot center dot O hydrogen bonds link adjacent spacing sheets to build a quasi-RP motif with a lower energy gap. In terms of ferroelectricity, the mixed-cation alloying has a crucial role in spontaneous polarization (P-s), as verified by structure analyses, quadratic optical nonlinearity, and electric hysteresis loops. The ordering of t-ACH(+) and EA(+) cations induces mmmFm symmetry breaking, along with an unusual in-plane P-s value of similar to 2.9 mu C/cm(2) in the ac plane. In combination with the anisotropic nature of its 2D motif, this ferroelectricity creates strong linearly polarized light sensitivity with a large dichroism ratio of similar to 3.2, far beyond those of most of the inorganic 2D systems. As far as we know, a 2D RP-type ferroelectric with a trans isomer cationic spacer is unprecedented, and the concept of reducing energy gaps via H-bonding interactions will strengthen the layered perovskite structure and shed light on the rational design of stable ferroelectrics toward photoelectric applications.

Automated summary

The study introduces hydrogen bonding to reduce van der Waals gaps in 2D RP-type perovskites, maintaining ferroelectricity. By incorporating a homoconformational trans isomer as a spacing cation, a quasi-RP motif with lower energy gap is achieved.