The crystal of [C(NH2)3]2Zn(SO4)2 guanidine zinc sulfate was grown and its structure, dilatometric, dielectric, elastic and piezoelectric properties were studied in a broad temperature range, covering the phase transition point. The crystal undergoes a continuous phase transition at 178 K from the room temperature tetragonal phase with a space group \(\text{I}\overline{4}2\text{d }\) to the tetragonal low temperature phase with a space group \(\text{I}\overline{4 }\) . The structural X-ray studies allowed proposing molecular mechanism associated with the rearrangement in the configuration of N–H⋯O hydrogen bonds and reorientation of guanidine cations in the structure, leading to a change in the symmetry of the low temperature phase. Results of thermal expansion and dielectric studies are typical of a structural nonferroelectric continuous transition. Also measurement of piezoelectric and elastic properties revealed small anomalies at 178 K. Below the transition temperature, a new piezoelectric component, that is a ferroelastoelectric macroscopic order parameter, was found.
