Thermodynamic energy density function for quantum paraelectric KTaO₃

KTaO₃ is a quantum paraelectric, which sits on the verge of the ferroelectric phase transition at 0 K yet remains paraelectric due to quantum fluctuations. While epitaxial strain can induce ferroelectricity, a thermodynamic description that quantifies the relationship between strain and ferroelectricity is currently missing for KTaO₃. In addition, limited experimental data exist on the ferroelectric transition temperatures for strained KTaO₃ thin films, which has made it challenging to establish a temperature-strain phase diagram. In this work, we construct a thermodynamic free-energy density function for KTaO₃ and determine its associated free-energy coefficients by fitting to experimentally measured thermodynamic properties. Using the thermodynamic model, we predict strain-induced ferroelectric phase transitions and construct a temperature-strain phase diagram. The thermodynamic model can be employed to guide the growth of strained KTaO₃ thin films to control the ferroelectric properties and serve as the basis for phase-field modeling of KTaO₃-based thin films and heterostructures.