Tailoring Potential Ferroelectric Properties in Conformationally Switchable Er(III)-Isothiocyanates Using Organic Cation Modulation

Hybrid molecular ferroelectrics necessitate switchable components, either organic or inorganic, capable of altering polarity under a reversing electric field. Isothiocyanate (NCS^–) ligands display such behavior through nonlinear coordination with metal ions. Homoleptic complexes of lanthanide ions exhibit variable coordination numbers, which can be controlled by the size of the counterions. We harnessed these properties to achieve polar order and ferroelectricity in hybrid [Er(III)(NCS)_x]^3‑x complexes. The incorporation of triethyl methylammonium (TEMA) cations yields the complex [TEMA]₄[Er(NCS)₇], which exhibits polarity at low temperatures with a Curie temperature (T_c) of 203 K. Notably, the use of bulkier and more rigid ethyltriphenyl phosphonium (ETPP) cations gave a room-temperature stable ferroelectric complex [ETPP]₃[Er(NCS)₆]. In contrast, flexible cations, such as tripropylmethylammonium (TPMA), tributylmethylammonium (TBMA), and tetraethyl phosphonium (TEP) ions, yielded only centrosymmetric complexes. The polar structural symmetries in [TEMA]₄[Er(NCS)₇] and [ETPP]₃[Er(NCS)₆] are attributed to pronounced distortions of the Er(III)-NCS coordination, driven by the rigid nature of organic counterions. The ferroelectric measurements on [ETPP]₃[Er(NCS)₆] gave a saturation polarization (P_s) of 1.6 μC cm^–2. Remarkably, [ETPP]₃[Er(NCS)₆] exhibits a high piezoelectric charge coefficient (d₃₃) of 22.7 pCN^–1 and an electrostrictive coefficient (Q₃₃) of 4.11 m⁴C^–2, enabling its application for piezoelectric energy harvesting.