Thermodynamic pathway between the non-polar and ferroelectric polymorphs of guanidinium ethoxysulfonate

At ambient pressure guanidinium etoxysulfonate (GES) crystallizes in two forms, -GES and -GES, in which the cations and anions are H-bonded into supramolecular 2D motifs. It has been shown that the non-polar -GES built of bilayers cannot be transformed into the ferroelectric -GES of a single-layer architecture, by a thermal stimulation. However, this transition can be accomplished under pressure. The transition is irreversible, of reconstructive-type, and proceeds with a rate depending on numerous factors like the pressure magnitude and duration, and the temperature variations. The slow-kinetics processes of transformation, once triggered by pressure, run a long time after the pressure releasing. The high-pressure dielectric studies revealed a complexity of the p‒T phase diagram of -GES. Three of the four new crystalline phases, which emerge under pressure between the ferroelectric and paraelectric phases, exhibit properties indicating a possible modulation of the crystal structure. The high-temperature phase I of -GES has been crystallized in situ in a glass capillary and its structure has been determined by single-crystal X-ray diffraction. The structural and calorimetric data evidenced that the transition between phases I and II of -GES has a reconstructive character, but it is fully reversible.