Perovskite-like materials exhibit desirable photophysical and electric properties that make them suitable for a remarkable breadth of applications in electronics and physics. In this contribution, we report on the multiphase ferroelectric and ferroelastic phenomena in a pyrrolidinium-based hybrid metal–organic material: (C4H8NH2)3[Sb2Cl9]. The title compound is the first pyrrolidinium derivative within the halobismuthates­(III) and haloantimonates­(III) families that is featured by the ferroelectric property. From a structural point of view, the crystal structure is built of [Sb2Cl9]3– ∞ perovskite-like layers, interdigitated by layers of pyrrolidinium cations. The rich solid-state dynamics of pyrrolidinium cations endowed (C4H8NH2)3[Sb2Cl9] with a complex sequence of temperature-dependent phase transitions. Remarkably, polar properties have been found to occur in all six phases, including room-temperature Phase I. Insights from variable-temperature single-crystal X-ray diffraction, dielectric spectroscopy, and T1 spin–lattice relaxation measurements revealed the general mechanism of most phase transitions, as related to the progressive ordering of nonequivalent pyrrolidinium cations. Noncentrosymmetry is probed by room-temperature second harmonic generation (SHG), while the ferroelectric property was evidenced through P(E) and dielectric measurements. The experimental values of spontaneous polarization were justified and analyzed in the context of theoretical values derived from quantum-chemical calculations. Optical measurements show that the integrity of the sample survives all of the phase transitions, despite sometimes significant deformations of the unit cell. The changes of symmetry associated with structural phase transitions are accompanied by an intriguing evolution of the ferroelastic domain structure with temperature.

The (C 2 H 5 NH 3 ) 2 [BiBr 5 ] (EBB) crystals adopt the one-dimensional (1D) polymeric anionic form [BiBr 5 ] ∞ 2− , which is preferred by halobismuthates(III) exhibiting polar properties and realized in R 2 MX 5 stoichiometry. Differential scanning calorimetry and dilatometric measurements reveal reversible structural phase transitions: at 160 K (phase I → phase II) and 120 K (phase II → phase III). The resolved crystal structures of EBB show the centrosymmetric space group in phase I (Aeam), polar (Pca2 1 ) in phase II, and polar (Aea2) in phase III. The presence of dielectric hysteresis loops in phases II and III evidence ferroelectric properties. The dielectric response [ε*(ω,T)] of EBB close to 160 K is characteristic of ferroelectrics with a critical slowing down process. The molecular mechanism of a paraelectric−ferroelectric phase transition at 160 K is explained as "order−disorder" (assigned to the dynamics of the ethylammonium cations) and dominating "displacive" (related to strong distortion of the 1D anionic network). The optical band gap obtained from UV−vis measurements is about 2.6 eV. The conduction band minimum is formed by the hybridized Bi 6p and Br 4p states. An analysis of the CSD results for haloantimonates(III) and halobismuthates(III) ferroelectrics characterized by [MX 4 ] − , [M 2 X 9 ] 3− , [MX 5 ] 2− , and [M 2 X 11 ] 5− anions is given.

Novel room-temperature ferroelectric crystals with complex sequences of phase transitions.

A brief description of the thermal, structural and dielectric properties of bis(ethylammonium) pentachlorobismuthate(iii) ferroelectric with Ps that equals to 1.4 μC cm−2 at 180 K is presented.