Ferroelectric and antiferroelectric phase transitions are critical phenomena in materials like PbTiO3 and HfO2-based films, governed by changes in polarization under external stimuli such as temperature, electric field, or strain. These transitions are distinct from non-polar phase changes, as they involve the reorientation or disappearance of electric dipoles. The Curie temperature, polarization switching dynamics, and thin-film constraints play pivotal roles in determining the functional properties of these materials.

In ferroelectric materials, the transition from a polar ferroelectric phase to a non-polar paraelectric phase occurs at the Curie temperature (Tc). For PbTiO3, Tc is approximately 490°C, while HfO2-based films exhibit Tc values that vary significantly with doping and strain, typically ranging between 400°C and 700°C. Above Tc, the spontaneous polarization vanishes due to thermal energy overcoming the dipole-dipole interactions that stabilize the ferroelectric phase. The transition is described by the Landau-Devonshire theory, where the free energy expansion explains the temperature dependence of polarization. In antiferroelectrics, such as PbZrO3 or modified HfO2, the transition involves the disappearance of antiparallel dipoles, leading to a paraelectric state at the antiferroelectric Curie temperature.

Polarization switching is a hallmark of ferroelectrics, where an external electric field reorients the spontaneous polarization between stable states. In PbTiO3, the switching process is influenced by domain wall motion, with typical coercive fields of 50-100 kV/cm. HfO2-based films, however, exhibit higher coercive fields (1-3 MV/cm) due to their smaller grain sizes and interface effects. The switching kinetics follow the Kolmogorov-Avrami-Ishibashi model, where nucleation and growth of domains dictate the polarization reversal time. Antiferroelectrics display double hysteresis loops under electric fields, with field-induced transitions to a ferroelectric phase before returning to the antiferroelectric state upon field removal. This behavior is exploited in energy storage applications, where the high recoverable energy density stems from the large polarization change during the transition.

Thin-film constraints drastically alter the ferroelectric and antiferroelectric properties compared to bulk materials. Epitaxial strain, caused by lattice mismatch with substrates, can enhance or suppress polarization. For example, compressive strain in PbTiO3 thin films on SrTiO3 increases Tc by up to 100°C, while tensile strain reduces it. HfO2-based films are particularly sensitive to interfacial layers and grain boundaries, which can stabilize the ferroelectric orthorhombic phase even in ultrathin (<10 nm) layers. Size effects also become prominent; below a critical thickness (∼2 nm for PbTiO3), depolarization fields can destabilize the ferroelectric phase. Antiferroelectric thin films face similar challenges, with the added complexity of maintaining the antiparallel dipole arrangement under finite-size effects.

The role of defects is another critical factor. Oxygen vacancies in HfO2-based films can pin domain walls, increasing coercive fields and reducing switchable polarization. In PbTiO3, defects often segregate at domain boundaries, influencing switching dynamics. Dopants are strategically used to tailor properties; for instance, Si doping in HfO2 stabilizes the ferroelectric phase, while La doping in PbTiO3 lowers Tc and modifies polarization.

Applications of these transitions are vast. Ferroelectric HfO2 is leveraged in non-volatile memory (FeRAM) due to its CMOS compatibility and scalability. PbTiO3-based systems are used in piezoelectric actuators and sensors, where the large electromechanical response is essential. Antiferroelectric HfO2 variants are promising for energy storage capacitors, with energy densities exceeding 50 J/cm³. The ability to control these transitions at the nanoscale opens avenues for ultra-low-power electronics and advanced functional devices.

Understanding the interplay between Curie temperature, polarization switching, and thin-film constraints is essential for optimizing material performance. Future research will likely focus on defect engineering and strain tuning to achieve room-temperature stability in ultrathin films, as well as exploring new material systems with lower energy barriers for switching. The continued development of these materials hinges on precise control over their phase transitions, enabling next-generation electronic and energy storage technologies.