Multiple Polymorphs and their Relationships in the Potential Ferroelectric Piperazinium Tetrafluoroborate

Symmetry-raising phase transitions are essential in the understanding of ferroelectric materials. Here, we investigate the complex polymorphism of piperazinium tetrafluoroborate, identified as a potential ferroelectric by our database search tool, FERROSCOPE. Using powder and single-crystal X-ray diffraction techniques, we have solved the structures of five polymorphs, two of which are newly observed. Space group choices were informed by variable-temperature second-harmonic generation measurements. We extend the structural family to piperazinium perrhenate and solve its room and low temperature structures. Group-subgroup analysis rationalizes the relationship between all phases including those of piperazinium perchlorate previously reported in the literature.