Crystal structures, phase transitions, and pressure-induced ferroelectricity in [C(NH2)3]5SO4(SO3-OC2H5)2F.

A guanidinium compound, C(NH(2))(3)SO(4)(SO(3)-OC(2)H(5))(2)F, with complex anionic sublattice has been synthesized and characterized by calorimetric and dielectric measurements at ambient and high hydrostatic pressures, as well as by single-crystal X-ray diffraction at varied temperatures. At room temperature, the crystal structure is orthorhombic, with the space group Pnma. In this phase, each of the two crystallographically nonequivalent ethoxysulfonate anions is disordered between two sites. On cooling, one of these anions starts to set in order at 228 K, where the crystal transforms in a continuous manner to the intermediate orthorhombic phase, with the space group P2(1)2(1)2(1). This transition belongs to the exceptionally rare pure gyrotropic phase transitions, the order parameter of which is described by the third-rank gyrotropic tensor. The ordering of the second ethoxysulfonate anion occurs suddenly at 187 K, inducing a first-order phase transition to the low-temperature phase of space group Pna2(1). The dissimilar response of both ethoxysulfonate anions to the temperature variation can be attributed to the different hydrogen bonding patterns they form with the cationic framework. Despite the polar symmetry, the low-temperature phase is not ferroelectric at ambient pressure, but it acquires ferroelectric features at elevated pressures above 140 MPa, as evidenced by the polarization reversal in an external electric field. The ferroelectric properties disappear on increasing pressure above 220 MPa, where the phase transition strongly modifying the crystal properties, but fully reversible, takes place. In the pressure-induced phase, a Debye-like dipolar relaxation process has been found and characterized as a function of pressure. The unusual properties of C(NH(2))(3)SO(4)(SO(3)-OC(2)H(5))(2)F under hydrostatic pressure have been summarized in the p-T phase diagram.