A thermodynamic potential and the temperature-composition phase diagram for single-crystalline K1-xNaxNbO3 (0 ≤ x ≤ 0.5)

Environment-friendly lead-free piezoelectric materials, such as the niobate perovskite compound (K1-xNaxNbO3), have attracted increasing attention over the last decade. Although extensive efforts have been made to study these materials experimentally, there have been much fewer theoretical studies on the microstructural evolution and property optimization of lead-free systems due to the lack of a comprehensive thermodynamic assessment. In this study, we first established a general procedure for constructing thermodynamic potentials for ferroelectric systems. We then developed an eighth-order Landau thermodynamic potential for K1-xNaxNbO3 (0 ≤ x ≤ 0.5) single crystals, taking into account the low temperature quantum mechanical effects. Based on this potential, we computed the spontaneous polarization and the dielectric constants for each composition as well as an overall temperature-composition phase diagram, all of which agree well with the available experimental data.