Structural instabilities, incommensurate modulations and P and Q phases in sodium niobate in the temperature range 300–500 K

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 (NaNbO 3 ) 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 NaNbO 3 .