**3.3 Ferroelectric properties**

Ferroelectric materials are a class of dielectric materials that exhibit ferroelectricity. Ferroelectricity is the ability of materials to possess spontaneous electric polarization and originates from a non-centrosymmetric crystal structure. The direction of spontaneous polarization can be reversed in accordance with an applied external electric field. Ferroelectric materials belonging to the perovskite family (ABX3 crystal structure) are a subclass of pyroelectric and piezoelectric materials. Ferroelectric materials show ferroelectric behavior only below the Curie temperature (TC). Above TC, these materials display the paraelectric state (i.e., they are only polarized under an applied electric field). For example, a well-known inorganic perovskite, BaTiO3, undergoes a structural transition from tetragonal to cubic above 393 K [35]. Recently, OIHPs have been studied specifically with a focus on their ferroelectric properties because of their structural transition and impressive dielectric properties. Although there is a debate on existing ferroelectricity in MAPbI3, several researchers have conducted theoretical and experimental investigations to search for evidence of ferroelectricity in OIHPs. For instance, Kutes et al. provided the first experimental evidence of ferroelectricity in solution-processed MAPbI3 thin films with a grain size of ∼100 nm by directly observing the ferroelectric domains through a PFM study, which is a necessary tool to observe the ferroelectric domains at nanometer resolution. The reversible switching of those ferroelectric domains was also realized by electrical poling with a DC bias [36]. Rakita et al. conducted an experimental

#### *Organic/Inorganic Halide Perovskites for Mechanical Energy Harvesting Applications DOI: http://dx.doi.org/10.5772/intechopen.105082*

investigation for the existence of ferroelectricity in tetragonal MAPbI3. They observed the polarization inversion under an external field, lack of inversion symmetry, and spontaneous polarization based on the measurements of a polarization-electric field (P-E) hysteresis loop, second harmonic generation signals, and pyroelectric response, respectively [37]. Kim et al. confirmed the tetragonal-to-cubic phase transition by measuring temperature-dependent dielectric properties [14]. These studies clearly indicate the experimental evidence for ferroelectricity in MAPbI3 films. In addition, the PFM study upon lead-free MASnI3 demonstrates its ferroelectric property (**Figure 2e–g**) [31]. From the PFM results, a well-defined butterfly-shaped hysteresis loop and the existence of 180° of domain switching validate ferroelectric polarization in MASnI3 films. However, commonly used 3D OIHPs including MAPbI3 and MASnI3 exhibit relatively low TC, limiting their wider applicability at high temperature. Pan et al. reported stable 3D (3-ammoniopyrrolidinium) RbBr3[(AP)RbBr3] perovskites synthesized by evaporation of the precursor's solution, which exhibited a ferroelectric nature at a high Tc = 440 K (**Figure 3**) [38].

Recently, researchers have been keen to study the ferroelectric properties of 2D OIHPs having advantageous characteristics including structural flexibility, diversity, and excellent moisture stability. The large asymmetric A-site cation

**Figure 3.**

*PFM analysis of 2D layered (ATHP)2PbBr4 films; a) lateral PFM phase, b) amplitude, c) corresponding topography images and d) obtained local piezoelectric response phase hysteresis (top) and amplitude (bottom) loops under applied DC-bias [34].*

provides an additional asymmetry to the 2D OIHP crystal. The orientation of such a large cation promotes ferroelectricity. Many excellent 2D OIHPs were designed and their ferroelectric properties investigated. For instance, 2D (ATHP)2PbBr4 was synthesized using the simple solution method and its ferroelectric property was investigated by measuring the P-E curve and lateral PFM [34]. The 180° contrast of domain orientation in the PFM phase image and separation of the adjacent domains by the domain walls in the PFM amplitude are a direct indictor of ferroelectricity for (ATHP)2PbBr4 (**Figure 4**). Another 2D OIHP [(4,4-DFHHA)2PbI4 (4,4-DFHHA=4,4-difluorohexahydroazepine)] displayed ferroelectric properties with a spontaneous polarization of 1.1 μC/cm<sup>2</sup> at room temperature, with a TC of 454 K [39].
