IMF2017

The poster deals with a member of the halide derivatives of diisopropylammonium abbreviated as DIPA. This family presents a practical interest due to its spectacular dielectric properties. Namely, DIPA bromide discovered simultaneously by two groups: Xiong R. –G. (Science, 2013, 339, 425.) and Jakubas R. (Cryst. Eng. Comm., 2013, 15, 940), possesses the spontaneous polarization about 23 μC·cm^-2 which is the highest among organic ferroelectrics and is comparable with classical inorganic ferroelectrics. The DIPA derivatives are, thus, attractive for further studies, and are potential candidates for superseding the lead-based toxic inorganic ferroelectrics which are widely used till now. The results obtained for iodide analog (DIPAI) are surprising and qualitatively different from those for the remaining compounds. Although the dielectric response is typical for a ferroelectric phase transition, when approaching the unique phase transition (T_c = 364 K) from below, the spontaneously polarized phase does not occur. Instead, the system transforms into another phase of a symmetry that is not group-subgroup related to the initial phase. A particular attention has been paid to the elucidation of the mechanism of the phase transition where in spite of a strong dielectric response a spontaneously polarized phase does not occur. The assembly of experimental data has allowed us to propose a general symmetry-based scheme of the related phenomena observed in the whole family of compounds. The proposed phenomenological Landau-type theory explains the mechanism of this behaviour in a unified way encompassing all the members of the diisopropylammonium family.