Structural and Phase Transition Studies of NCS- Based 3d Metal Organic–Inorganic Hybrid Ferroelectrics

Abstract

Organic–inorganic hybrid isothiocyanate frameworks are promising candidates for ferroelectric and dielectric applications owing to their structural tunability, molecular dipoles, and temperature-dependent phase transitions. In this study, we investigate trimethylsulfonium-stabilized 3d metal thiocyanate hybrids, tetrakis(trimethylsulfonium) hexaisothiocyanatomanganese (TMS_MnNCS) and tetrakis(trimethylsulfonium) hexaisothiocyanatonickel (TMS_NiNCS). Single-crystal X- ray diffraction shows that TMS_MnNCS and TMS_NiNCS crystallize in polar non- centrosymmetric space groups at low temperature (Pn and P2₁, respectively) and transform to centrosymmetric P2₁/n symmetry near room temperature. Variable- temperature powder X-ray diffraction and differential scanning calorimetry reveal multiple thermally driven structural transitions in the 3d analogues, with TMS_MnNCS displaying a pronounced transition near ~239 K and additional events around ~337 K, while TMS_NiNCS exhibits transitions near ~159 K and ~354 K. Thermogravimetric analysis indicates good thermal stability up to ~405 K (Mn) and ~364 K (Ni), and optical measurements show semiconducting band gaps of ~2.13 eV and ~3.02 eV, respectively. Low-temperature polarization–voltage measurements for TMS_MnNCS exhibit clear ferroelectric hysteresis (Ps ≈ 20 μC cm⁻², Pr ≈ 14.7 μC·cm⁻²), confirming switchable polarization in the polar phase. These results demonstrate that bulky trimethylsulfonium cations effectively stabilize non-centrosymmetric NCS-based metal frameworks across both 3d and 4f systems, establishing a versatile platform for temperature-responsive ferroelectric, piezoelectric, and nonlinear-optical materials. This discovery tells us about trimethylsulfonium cations and the key role it plays in controlling polar phases and improving how the structure of NCS-based hybrid ferroelectric materials influence their properties