First-principle study of strain-driven phase transition in incipient ferroelectric SrTiO3

First-principle calculations were carried out to investigate the epitaxial strain dependence of the structural stability and related properties of SrTiO₃. The P4mm-to-P4/mmm-to-Amm2 phase transitions in SrTiO₃ with the epitaxial strain changing from compressive to expansive have been identified by analysis of the soft-mode eigendisplacement. The zero-temperature critical strains for the P4mm-to-P4/mmm and P4/mmm-to-Amm2 phase transitions are −0.72% and 0.83%, respectively, which is consistent with the previous predictions based on the phenomenological thermodynamic model and the parameterized total-energy model. The influences of the epitaxial strain on the dielectric tensor and spontaneous polarization along the phase transition path are discussed in terms of charge transfer. Further investigations on the vibration modes show unusual softening behaviors of the A₁ and B₂ modes in the Amm2 phase, which may be associated with the soft phonon modes away from Γ point and possibly result in other phase transitions unreported by any previous studies.