Zero-Dimensional Molecular Ferroelectrics with Significant Nonlinear Effect and Giant Entropy

The search for ferroelectric materials that have a significant progress in applications, such as ferroelectric field-effect transducers, piezoelectric sensors, nonlinear optical devices, electrocaloric refrigeration, and so forth, remains a great challenge. Here, we report the discovery of two unique molecular ferroelectrics (TMP)(2)(SbX5)(SbX3) (TMP = thiomorpholine = (CH2)(4)NH2S+, X = Cl (1-Cl), Br (2-Br)), which are the first reported examples of halogenoantimonates(III) and halogenobismuthates(III) with R-2(MX5)(MX3)-type zero -dimension (0D) configuration ascribing to the discrete SbX5 and SbX3 polyhedrons separated by the TMP cations. (TMP)(2)(SbX5)(SbX3) also feature the highest phase transition temperature (Tc, in which 362/442 K for 1-Cl and 385/477 K for 2-Br) and giant zero -field entropy change (131.68/109.16 J mol(-1) K-1 for 1-Cl and 183.24/150.32 J mol-1 K-1 for 2-Br) in halogenoantimonates(III) and halogenobismuthates(III)-based molecular ferroelectrics. A low thr-coordinated SbX3 component accelerates the crystallization of (TMP)(2)(SbX5)(SbX3) in the noncentrosymmetric structure because it breaks the R(2)MX5-type construction. Thus, they exhibit significant room-temperature SHG signals (in which 4.2 x KDP for 1-Cl and 3.8 x KDP for 2-Br) and polarization-dependent SHG response. In addition, ultraviolet-visible absorption and photoluminescence spectra reveal that the band gap is successfully tuned from 3.366 to 2.600 eV for 1-Cl to 2-Br, while emission at 545 nm for 1-Cl is redshifted to 611 nm for 2-Br; both large Stokes shifts and broadband emission characterize the PL of 0D metal halide hybrids and are attributed to the strong coupling of the excitons to multiple phonons. This work will provide an avenue to explore the SHG responses and photoelectric characteristics of low dimensional halogenoantimonates(III) and halogenobismuthates(III)-based molecular ferroelectrics.