Origin of Metastable Properties in the Ferroelectric Phase of Tetraguanidinium Dichloro-Sulfate

Single-crystal X-ray diffraction, calorimetry, and temperature- and time-dependent Raman spectroscopy have been employed to study anomalous properties of ferroelectric tetraguanidinium dichloro-sulfate. A clearly metastable character of the changes in the Raman spectrum has been ascribed to the structural features of the crystal in the intermediate phase II. This phase of space group Fmm2 is strongly disordered, both in cationic and anionic sublattices, which results in a large number of configurations with local energy minima. This implies a quasistatic or long-time relaxation character of the configurations, which are separated by an energy barrier from the state corresponding to the global energy minimum in the room-temperature ferroelectric phase III. These features play a key role in the slow kinetics of the II/III phase transition. The structural evidence of the guanidinium cations disordering in phase II is very unusual because of the disordering which develops in low temperature from the high-temp...