From nonpolar to ferroelectric crystal structure: the temperature-tuned growth of two guanidinium ethoxysulfonate polymorphs

Guanidinium ethoxysulfonate, C(NH2)3]+C2H5O-SO3]-, was synthesized, and two polymorphs, both stable at normal conditions, were grown from an aqueous solution by only a slight change in the crystallization temperature. The nonpolar polymorph I is built of hydrogen-bonded bilayers, while the ferroelectric polymorph II consists of single-layers. The diversity in the crystals' architecture and properties originates from the excessive number of proton-acceptor sites. At 298 K, the structure of polymorph I is orthorhombic, space group Pbam, formed of supramolecular hydrogen-bonded sheets. Within such a sheet, the ethoxysulfonate anions assume alternately cis and trans conformations, both disordered at room temperature and at 150 K. The anisotropy of the crystal structure is mirrored by a strong anisotropy of its thermal expansion. Upon cooling at 120 K, the crystal undergoes a first-order order-disorder phase transition. The structure of polymorph II is also reinforced by the two-dimensional network of NH...O hydrogen bonds, but the supramolecular motif formed is different from that of polymorph I. The H-bonded strongly corrugated sheets are stacked, forming a densely packed single-layer structure. All the anions assume the same trans conformation. At 298 K, they are disordered between the two sites related by the mirror symmetry plane. The onset of ordering of the anions coincides with the Curie point at TC = 211 K, at which the dielectric constant exceeds 4000. The continuous paraelectric-ferroelectric phase transition is associated with the symmetry change Pnma --> Pna21. Despite the apparent order-disorder character of the transition, the transition mechanism also involves a substantial displacement of the ions and a rearrangement of the H-bonded network.