Photoinduced Local Symmetry Breakage in SrTiO3 and Potential Pathways to Ferroelectricity

During the past few years Strontium Titanate, STO, has been the subject of extensive studies exploring the phase transitions it undergoes from paraelectric to ferroelectric or antiferrodistortive phases, induced by electromagnetic radiation such as lasers. This ability to exhibit photoinduced ferroelectricity has profound implications for its potential applications in photovoltaic and optoelectronic devices. Additionally, SrTiO₃ exhibits a rich dielectric behavior under UV irradiation in the presence of an electric field. In this work, we explore the nature of the photoinduced enhancement of the STO dielectric constant under UV irradiation. Analyzing the STO potential energy surface (PES), the first excited states exhibit a local breaking of symmetry due to the displacement of the Ti atom off the center of the unit cell. Furthermore, two key variables linked to the ferroelectric phase transition in STO, namely, pressure and oxygen vacancies, are investigated. Pressure can modify the ground state and excited state PES, tailoring the depth of the excited state well or reducing the energy gap between the ground and excited states. The distribution of oxygen vacancies in the STO bulk was also explored, revealing preferential configurations and the presence of aligned Ti³⁺-O_vac-Ti³⁺ or Ti³⁺-O_vac complexes. These configurations can lead to a small polarization of the material, which could serve as a driving force to align the off-center photoexcited Ti polarons.