Thermodynamic Theory of Strained Thin Films of Incipient Ferroelectric KTaO₃

KTaO₃ is an incipient ferroelectric with quantum fluctuations stabilizing the soft phonon mode at low temperatures. The influence of external perturbations from strain, dopants, or stress can, however, be used to induce a ferroelectric state. A thermodynamic theory for KTaO₃ based on the Landau-Ginzburg-Devonshire theory of ferroelectrics has not been established yet. In this study, we formulate a thermodynamic free energy density function for KTaO₃ and use it to describe the thermodynamics of KTaO₃ single crystals and strained KTaO₃ thin films. Using the established Landau coefficients, the calculated electric field-induced polarization and dielectric constant show a good agreement with experimentally measured values. The stable ferroelectric phases in the analytically constructed temperature-strain phase diagram are identified using the phase-field method. The thermodynamic model developed in this work for strained KTaO₃ thin films can not only be employed to guide their experimental synthesis and growth but also be utilized in the phase-field method to model domain morphologies.