The extraordinary thermal stability of [NH₂CHNH₂]₃Bi₂I₉ (FA₃Bi₂I₉) crystals, up to around 600 K, has been enhanced by using the evaporating method. DSC measurements have revealed reversible structural phase transitions: at 203 K (Phase I → Phase II), 173 K (II → III), 134.7 K (III → IV), 131.4 K (IV → V), and 120 K (V → VI). The crystal structures of FA₃Bi₂I₉ adopt the polar space groups P6₃mc at 220 K (Phase I) and Cmc2₁ at 190 K (Phase II). The pyroelectric current (I_pyr) measured in the temperature range covering all the phases of FA₃Bi₂I₉ confirmed their polar nature. The reversibility of the spontaneous polarization in each phase has been confirmed by the observed hysteresis loops. All the phase transitions are dielectrically active. The dielectric response (ε*(ω,T) close to 134.7 and 131.4 K) is characteristic of ferroelectrics with a critical slowing-down process. Optical bandgaps estimated from the UV-vis measurements and calculated using the DFT method are equal to 1.85 and 1.89 eV, respectively. On the basis of the structural, dielectric, and spectroscopic results, the molecular mechanisms of the phase transitions have been proposed.

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.

We demonstrate that a surface resonance (pseudosurface wave) may transform into a true surface wave, i.e., acquire an infinite lifetime, at a single isolated point within a bulk band (radiative region) in a model of a stressed auxetic material. In contrast with the secluded supersonic elastic surface waves, the one found here does not belong to a dispersion line of true surface waves. Therefore we propose to call it an isolated true surface wave (ITSW). The ITSW manifests itself by a deltalike peak in the local density of states and by anomalies in reflection coefficients. The phenomenon may be useful in redirecting energy and/or information from the bulk to the surface in devices supporting guided acoustic waves.