Two hybrid crystals imidazolium iodoantimonate(III) and iodobismuthate(III), (C₃H₅N₂)₃[Sb₂I₉] (ImIA) and (C₃H₅N₂)₃[Bi₂I₉] (ImIB), have been synthesized and characterized in a wide temperature range (100–350 K) by means of X-ray diffraction, dielectric spectroscopy, proton magnetic resonance (¹H NMR), FT-IR spectroscopy and optical observations. They undergo two temperature induced solid–solid structural phase transitions. The first one, quasi-continuous (with temperature hysteresis below 1 K), occurs at 324 K in ImIA and 327 K in ImIB, and the second one, clearly of the first order, at 273/278 (cooling/heating) and 291/295 K, in ImIA and ImIB, respectively. Ferroelastic properties are maintained in low-temperature phases. Both materials are isomorphic in the corresponding phases. High temperature phase I has a hexagonal P6₃/mmc symmetry, and phase II has orthorhombic Cmcm. The crystal architecture is composed of discrete, face-sharing bioctahedra [M₂I₉]³⁻ (M: Sb, Bi) and imidazolium cations which are highly disordered over phases I and II. The dynamics of the imidazolium cations has a prominent impact on the stability of the particular phases.

The real part of the complex electric permittivity at low frequencies and at several biasing fields (between 0 and $5\times {10}^5\mathrm{V}∕\mathrm{m}$) has been measured in ferroelectric crystals ${\left[\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\right]}_5{\mathrm{Bi}}_2{\mathrm{Cl}}_{11}$ (MAPCB) and ${\left[\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\right]}_5{\mathrm{Bi}}_2{\mathrm{Br}}_{11}$ (MAPBB) in the temperature range covering the temperature of the ferroelectric phase transitions. Comparative measurements for the known triglycine sulphate ${\left(\mathrm{N}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}\right)}_3{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ crystal have been used as a test of the validity and of possible errors in the determination of the ferroelectric equation of state by the method applied. The estimates of the critical parameters $T_C$, $\gamma$, and $\delta$ then have been evaluated for MAPCB and MAPBB on the basis of the Widom-Griffiths scaling hypothesis. Complementary pyroelectric measurements of the spontaneous polarization providing the critical exponent $\beta$ are in accordance with the parameters obtained.

Crystals of diisopropylammonium iodide are synthesized, grown and characterized. Two phases: P2₁/m (Z = 1) and P2₁2₁2₁ (Z = 2) are observed. In contrast with analogous compounds no polar phase occurs, despite a critical-like electric behaviour. A phenomenological theory is proposed to describe the thermodynamics of the whole family of diisopropylammonium halides.

Dielectric polarization and electric polarization of electrodes are the common features of polar materials. We described methods to analyze their contributions and showed that both dependencies on temperature of dielectric conductivity and electrode polarization and the exponents characterizing these dependencies are excellent markers of phase transitions. Proposed methods were applied to several compounds, such as liquid crystals, pharmacological compounds, monoalcohols, polyalcohols, and various thermodynamic phases. Common behavior was noted for materials under study. In similar phases, various substances have the same values of the exponents characterizing electric conductivity and contribution from the electrode polarization. These exponents show discontinuities at phase transition temperatures between crystal-like and liquid-like phases.