Structural Disorder and Molecular Dynamics in Ferroelectric Crystals [C(NH2)3]4Br2SO4 and [C(NH2)3]4Cl2SO4

Two isomorphic ferroelectric crystals of tetraguanidinium dibromo-sulfate, [C(NH2)3]4Br2SO4, and tetraguanidinium dichloro-sulfate, [C(NH2)3]4Cl2SO4, have been studied by Raman spectroscopy, single-crystal X-ray diffraction, and scanning calorimetry. Despite the isostructural paraelectric and quasi-isostructural ferroelectric phases, the crystals show distinct phase relations and properties. The differences in the temperature evolution of Raman bands are discussed in terms of thermally activated molecular motions. These results, supported by the temperature-dependent structural and calorimetric data, evidenced that tiny differences in the crystal packing, as well as in the number and strength of hydrogen bonds, are the main factors that determine the crystal thermodynamic behavior and properties. Slightly more loose packing of [C(NH2)3]4Br2SO4 results in the structural disorder activated well below the transition point and contributes to the fact that thermodynamically first-order transition acquires some...