The Role of Reversible and Irreversible Contributions on Structure–Property Relations in Sn Doped BaTiO3

To meet the growing demand for highly efficient, multifunctional, and low-toxicity energy conversion systems, lead-free perovskite materials have gained significant attention as promising candidates. Light-active polar perovskite oxides such as BaSnₓTi₁₋ₓO₃ are particularly promising due to their structural flexibility and chemical tunability. These materials exhibit the ability to convert, for example, mechanical energy or light into electrical energy, and enable light-driven strain or photocatalytic reactions. These coupled functionalities are made possible by a low band gap, which enables energy conversion over a broad range of the visible spectrum, combined with switchable spontaneous polarization. However, these materials face challenges in photovoltaic applications, including low photovoltage and quantum efficiency. The substitution of B-site cations with non-d0 metal cations offers a promising strategy to tune the band gap of polar perovskite oxides and modify their photophysical properties. Sn doping of BaTiO₃ were demonstrated to yield above-band gap open-circuit voltages and internal electric fields arising from its ferroelectric nature, which facilitate efficient charge carrier separation and enhance the photoferroelectric response. Having said this, there are discrepancies in the understanding of the effect of Sn on properties, such as the remanent polarization, the band gap, and piezoelectric charge coefficient. Current observations have primarily focused on the room temperature behavior of BaTiO₃ with varying Sn content, revealing inconsistencies in the material’s behavior and no clear emerging trends. These discrepancies can be approached by conducting detailed temperature-dependent studies. These reveal, for instance, the impact crystal structure changes have, by uncovering the effects of phase boundaries, and the transition from ferroelectric to paraelectric phase. Focusing on the reversible (e.g., lattice deformations) and irreversible (e.g., domain wall mobility) contributions of dielectric and piezoelectric response further enables understanding the impact Sn has on the electromechanical properties. Moreover, domains and domain wall mobility significantly influence the photoferroelectric response, particularly near phase boundaries where an increased domain density can enhance photovoltaic performance. Hence, correlating structure-property relations is another important tool to advancing knowledge of the photoferroelectric behavior. Sn doping in BaTiO₃ showed to alter the crystal phase and induce phase coexistence. Further, the grain size, domain size and domain structure were also modified by Sn. These structural changes significantly influenced reversible and irreversible contributions and maximized the ferroelectric and piezoelectric performance at specific Sn-content and temperatures. In the context of photoferroelectrics, Sn doping has shown to slightly increase the bandgap. These findings contribute to a deeper understanding of polar perovskite oxides, paving the way for their application in next-generation energy conversion systems.