Thermomechanical Stabilization of Electron Small Polarons in SrTiO₃ Assessed by the Quasiharmonic Approximation

Abstract

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.