First-principles Study of Anomalous Electromechanical Properties Hidden in Paraelectric SrTiO<sub>3</sub>: Simultaneous Emergence of High-stiffness and Superelasticity via Strain-induced Ferroelectric Phase Transition

Since the recent discovery of superelastic behavior in ferroelectric BaTiO3, understanding the mechanical and electromechanical properties of ferroelectric ceramics under high loading conditions has become crucial for engineering applications such as advanced functional nanodevices. Here, we shed light on paraelectric SrTiO3 in contrast ot ferroelectric BaTiO3, and investigate the mechanical strength, deformation behavior, and electromechanical response of the paraelectric ceramics SrTiO3 under high stress conditions with ferroelectric phase transition. Under low-strain conditions, that is, in the paraelectric phase, SrTiO3 shows mechanical properties that maintain the characteristic high stiffness of ceramics. On the other hand, after the strain-induced ferroelectric phase transition, we find that SrTiO3 demonstrates superelastic-like nonlinear deformation behavior. Therefore, SrTiO3 exhibits both high stiffness and deformability simultaneously due to the strain-induced ferroelectric phase transition. Additionally, we clarify the emergence of hysteresis loops in both mechanical deformation and electromechanical responses without accompanying structural phase transitions. We also reveal that giant piezoelectric coefficients manifest at discontinuous points of polarization curves. We conclude that these unique properties in SrTiO3 originate from the displacement of oxygen atoms and the chemical bonding network due to the ferroelectric phase transition, based on analysis of the electronic structure and interatomic distances of each atom with respect to strain. Our results advance fundamental understanding of SrTiO3 behavior under extreme loading conditions and establish a foundation for advanced engineering applications.