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

Abstract

A new ferroelectric, ${[\mathrm{C}{(\mathrm{N}{\mathrm{H}}_{2})}_{3}]}_{4}{\mathrm{Br}}_{2}\mathrm{S}{\mathrm{O}}_{4}$, 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 ${T}_{C}=365.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The transition is driven by disordering of the $\mathrm{S}{\mathrm{O}}_{4}^{2\ensuremath{-}}$ 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 ${[\mathrm{C}{(\mathrm{N}{\mathrm{H}}_{2})}_{3}]}_{4}{\mathrm{Cl}}_{2}\mathrm{S}{\mathrm{O}}_{4}$ crystal it has been shown that the $\mathrm{Br}∕\mathrm{Cl}$ 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 ${[\mathrm{C}{(\mathrm{N}{\mathrm{H}}_{2})}_{3}]}_{4}{\mathrm{Cl}}_{2}\mathrm{S}{\mathrm{O}}_{4}$ and ${[\mathrm{C}{(\mathrm{N}{\mathrm{H}}_{2})}_{3}]}_{4}{\mathrm{Br}}_{2}\mathrm{S}{\mathrm{O}}_{4}$ are isostructural in tetragonal space groups $I\overline{4}2m$, below ${T}_{C}$ the first one becomes a two-directional ferroelectric in space group $Cmc{2}_{1}$, 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 ${[\mathrm{C}{(\mathrm{N}{\mathrm{H}}_{2})}_{3}]}_{4}{\mathrm{Br}}_{2}\mathrm{S}{\mathrm{O}}_{4}$, can be useful for electronic applications.