Thermal and x‐ray diffraction measurements on compositions from the system NaxK1−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%.

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