Thermal and X‐Ray Diffraction Studies of the NaNbO₃–KNbO₃ System

Thermal and x‐ray diffraction measurements on compositions from the system Na_xK_1−xNbO₃ have revealed the presence of phase boundaries at approximately 82.5, 67.5, and 52.5 mole × NaNbO₃ at 25°C, in addition to one very near the NaNbO₃ end member. The optimum piezoelectric properties in this system are apparently associated with compositions near the latter boundary. Calorimetric determinations of the enthalpy and entropy changes associated with the ferroelectric→ferroelectric and ferroelectric→paraelectric transformations in the system clearly revealed the presence of the phase boundaries while measurements of the transformation temperatures alone did not readily reveal the boundaries. Two different superlattice perovskite phases were observed in the system, one existing in the region ∼0.98≥x≥0.825 and another in the region 0.825≥x≥0.675. No multiple‐cell structures were observed in the system at NaNbO₃ concentrations below about 70 mole%.