Thermomechanical stabilization of electron small polarons in SrTiO3 assessed by the quasiharmonic approximation

We predict a predominance diagram for electron defects in the temperature-hydrostatic stress space for SrTiO₃ by combining density functional theory and the quasiharmonic approximation. We discovered two regimes where small polarons dominate: under tensile stress at lower temperature due to a larger relaxation volume of the defect Ω, and under compressive stress at higher temperature due to a smaller Ω and larger formation entropy. This provides a means to modulate the electronic conductivity via controlling the underlying charge carrier. Furthermore, the results challenge the common association between larger Ω and charge localization by demonstrating that at high temperature the free electron can induce larger Ω compared to the small polaron. This finding is attributed to the ability of the free electron to generate greater vibrational entropy upon finite isothermal expansion.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (DMR-1419807)National Science Foundation (U.S.) (TG-DMR 140065