Ferroelectricity in metal halide perovskites, particularly methylammonium lead iodide (MAPbI3), has emerged as a subject of intense research due to its unique interplay of structural dynamics and electronic properties. The ferroelectric behavior in these materials arises from the non-centrosymmetric arrangement of ions, coupled with the dipolar nature of organic cations, enabling spontaneous polarization that can be switched under an external electric field. This article examines the mechanisms of polarization switching, domain wall dynamics, and piezoelectric responses in ferroelectric perovskites, with a focus on the role of organic cations and phase transitions.

The origin of ferroelectricity in MAPbI3 is closely tied to the orientation of the methylammonium (MA+) cation within the inorganic PbI6 octahedral framework. At room temperature, the MA+ cation exhibits dynamic disorder, rotating freely within the perovskite cage. However, below the tetragonal-to-orthorhombic phase transition at approximately 160 K, the MA+ cations align along specific crystallographic directions, breaking inversion symmetry and inducing a net dipole moment. This alignment is responsible for the spontaneous polarization observed in ferroelectric phases. The polarization direction is influenced by hydrogen bonding between the MA+ cation and the iodide ions, which stabilizes specific orientations and contributes to the anisotropic ferroelectric response.

Polarization switching in MAPbI3 occurs through the reorientation of MA+ dipoles under an applied electric field. The switching process involves the collective motion of organic cations, which is coupled to distortions of the PbI6 octahedra. The energy barrier for dipole reorientation is estimated to be around 60 meV, indicating relatively low coercive fields compared to conventional oxide ferroelectrics. The switching speed is limited by the rotational dynamics of the MA+ cation, with characteristic timescales on the order of picoseconds at room temperature. Hysteresis loops measured via piezoresponse force microscopy (PFM) reveal a remanent polarization of approximately 0.1 to 1 μC/cm2, depending on crystallographic orientation and sample quality.

Domain walls in ferroelectric MAPbI3 exhibit distinct characteristics due to the hybrid organic-inorganic nature of the material. These walls are typically several nanometers wide and are highly mobile, driven by the flexibility of the PbI6 framework and the weak bonding between MA+ cations and the inorganic lattice. Two primary types of domain walls are observed: 180° walls, where the polarization reverses direction, and 90° walls, where the polarization vector rotates by 90 degrees. The energy density of these walls is lower than in oxide perovskites, ranging from 1 to 10 mJ/m2, due to the softer lattice and weaker electrostatic interactions. Domain wall motion is influenced by local strain and defects, which can pin the walls and affect switching dynamics.

The piezoelectric response of MAPbI3 is directly linked to its ferroelectric properties. The coupling between polarization and strain arises from the distortion of PbI6 octahedra and the displacement of MA+ cations under mechanical stress. The effective piezoelectric coefficient (d33) is reported to be in the range of 10 to 50 pm/V, with significant anisotropy between different crystallographic directions. This response is highly sensitive to temperature, with a pronounced increase near phase transitions due to enhanced lattice softness. The piezoelectric effect is also influenced by domain wall motion, as the reconfiguration of domains under stress contributes to the overall strain response.

Phase transitions play a critical role in modulating ferroelectric and piezoelectric properties. MAPbI3 undergoes multiple structural transitions as a function of temperature, including cubic-to-tetragonal at 330 K and tetragonal-to-orthorhombic at 160 K. These transitions are accompanied by changes in the dynamics of the MA+ cation and the distortion of the PbI6 framework. The ferroelectric phase is stable only in the orthorhombic and tetragonal phases, where the MA+ cations exhibit ordered or partially ordered arrangements. In the cubic phase, the MA+ cations are fully disordered, leading to a paraelectric state. The transition temperatures are sensitive to external pressure and chemical substitutions, which can be used to tune the ferroelectric properties.

The role of organic cations extends beyond dipole formation, as they also influence lattice dynamics and defect chemistry. The MA+ cation introduces additional vibrational modes that couple to the inorganic lattice, affecting phonon dispersion and thermal conductivity. Hydrogen bonding between the MA+ cation and iodide ions introduces local strain fields that can stabilize polar nanoregions even in the absence of long-range ferroelectric order. Substituting the MA+ cation with larger organic cations, such as formamidinium (FA+) or cesium (Cs+), alters the phase transition behavior and ferroelectric stability. For example, FAPbI3 exhibits a lower phase transition temperature and weaker ferroelectric response due to the reduced dipole moment and increased rotational entropy of the FA+ cation.

Defects and impurities significantly impact ferroelectric properties by pinning domain walls and creating local electric fields. Iodide vacancies, which are common in MAPbI3, act as charged defects that can screen polarization and hinder switching. Grain boundaries and interfaces also play a role, as they can nucleate domains with different polarization orientations. The interaction between defects and ferroelectric domains is complex, with some defects promoting domain wall pinning while others facilitate nucleation of new domains.

In summary, ferroelectricity in MAPbI3 and related perovskites is governed by the interplay between organic cation dynamics, inorganic lattice distortions, and phase transitions. The polarization switching behavior, domain wall mobility, and piezoelectric responses are distinct from those of conventional ferroelectrics due to the hybrid nature of these materials. Understanding these properties requires careful consideration of the roles played by organic cations, structural phase transitions, and defects. Future research may explore chemical substitutions and strain engineering to further tailor ferroelectric and piezoelectric responses for specific applications.