Thermodynamic evaluation and hydrothermal preparation of KxNa1−xNbO3

The Gibbs free energies of reaction ΔrGT of KNbO3 and NaNbO3 were calculated and then that of KxNa1−xNbO3 was estimated. On the basis of the thermodynamic calculation results, the hydrothermal temperatures were designed at 100, 160, and 230°C respectively. However, NaNbO3 was prepared when the heating temperature was higher than 160°C, and KNbO3 and KxNa1−xNbO3 were obtained at 230°C, suggesting that only at a higher temperature the kinetic requirement is satisfied. According to the X-ray diffraction results, both the solid solutions, NaNbO3-based solid solution, Na1−xKxNbO3, and KNbO3-based solid solution, K1−xNaxNbO3, were hydrothermally synthesized in 6 mol/L alkali (NaOH and KOH) solution at 230°C.