An optical, dielectric, and structural study has been carried out on NaNb${\mathrm{O}}_3$ and the solid solution (Na,K)Nb${\mathrm{O}}_3$, both single crystals and ceramics being used. No evidence for ferroelectricity in NaNb${\mathrm{O}}_3$ was found, and the crystal seems to be antiferroelectric in accordance with the nonpolar structure reported by Vousden. It is shown that a small addition of KNb${\mathrm{O}}_3$ to pure NaNb${\mathrm{O}}_3$ produces a new ferroelectric phase, the existence of which suggests a possible explanation of the conflicting dielectric and structural properties previously reported.

The phase diagram of NaNb${\mathrm{O}}_3$-KNb${\mathrm{O}}_3$ is given. This, together with the optical and x-ray studies of pure NaNb${\mathrm{O}}_3$, shows that the three phase transitions in NaNb${\mathrm{O}}_3$ are quite different in nature from the BaTi${\mathrm{O}}_3$-type transitions in KNb${\mathrm{O}}_3$.

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