Effect of halogen atom exchange on the thermodynamic behavior and ferroelectric properties of [C(NH2)3]4Br2SO4

A new ferroelectric, [C(NH2)(3)](4)Br2SO4, has been characterized by dielectric, calorimetric and dilatometric methods, and its paraelectric and ferroelectric structures determined by x-ray diffraction. The crystal undergoes the first-order ferroelectric-paraelectric phase transition at T-C=365.5 K. The transition is driven by disordering of the SO42- anions, but the spontaneous polarization originates essentially from the ionic displacements. The temperature dependence of the order parameter is well described by Landau's mean-field approximation. By comparing with [C(NH2)(3)](4)Cl2SO4 crystal it has been shown that the Br/Cl halogen atom exchange caused the anisotropic deformation of the crystal lattice modifying the ionic and hydrogen-bond interactions, resulting in the upward shift of the Curie point, the essential changes in the crystal phase diagram, and modified ferroelectric properties. Although the paraelectric phases of [C(NH2)(3)](4)Cl2SO4 and [C(NH2)(3)](4)Br2SO4 are isostructural in tetragonal space groups I (4) over bar 2m, below T-C the first one becomes a two-directional ferroelectric in space group Cmc2(1), while the second one a one-directional ferroelectric in space group Fmm2. The room-temperature ferroelectricity, and the high piezoelectric response and large electric field-switchable strain, revealed in [C(NH2)(3)](4)Br2SO4, can be useful for electronic applications.