The relation between the phase transition in the vicinity of 420 K discovered earlier and the formation of an incommensurate phase in the temperature range 410-460 K is established based on the X-ray, dielectric, and dilatometric studies of sodium niobate (NaNbO3) single crystals and ceramics. It is also established that this phase is characterized by temperature and temporal instabilities. Anomalies in some physical characteristics in the vicinity of 350 K are revealed. It is also shown that the thermodynamic history of the samples is important for the coexistence of the regions of the ferroelectric Q and antiferroelectric P phases in NaNbO3.
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ACKNOWLEDGMENTS This study was supported by the Russian Foundation for Basic Research, project no. 99-02-17575.