Dynamic control of halide perovskite structures for tailored ferroelectric and second-order nonlinear optical functionalities.

Metal halide perovskites (MHPs) are rapidly developing as a class of versatile materials combining the exceptional optoelectronic characteristics with tunable ferroelectricity and nonlinear optical responses. Spanning across the three-dimensional, two-dimensional, and one-dimensional architectures, these materials have demonstrated exceptional structural diversity, providing immense opportunities for tailored property design. We start by referencing the classic oxide perovskites, sharing differences and similarities of these material systems. The fundamental mechanisms driving ferroelectricity in MHPs range from displacive distortions and lone-pair stereoactivity to organic cation ordering. The ability to control these mechanisms through precise organic cation site (A-site) and metal site (B-site) engineering, dimensionality tuning, and external stimuli opens new avenues for designing high-performance ferroelectric and second-harmonic generation (SHG)-active materials. This review highlights the rich structural diversity in halide perovskites, with a focus on the critical structure-property relationships that govern their ferroelectric and nonlinear optical behaviors. We discuss key design strategies that exploit asymmetric coordination, excitonic channels, and resonance effects to enhance SHG responses and polarization switching. Furthermore, we analyze how these materials might be included into useful devices such as ferroelectric photovoltaics and photodetectors, as well as how they could be used in multipurpose optoelectronic applications. Finally, we discuss the stability and scalability issues that MHP ferroelectrics are having in the commercialization process and provide insights for future research avenues that may help realize these materials' full potential.