Phase transitions of sodium niobate powder and ceramics, prepared by solid state synthesis

Phase transitions of sodium niobate, prepared by the solid state synthesis method, were examined using dielectric measurements, differential scanning calorimetry, and high temperature x-ray diffraction, in order to contribute to the clarification of its structural behavior below $400°C$⁠. Four phase transitions were detected in the ceramic sample using dielectric measurements and differential scanning calorimetry and the obtained temperatures were in a good agreement with previous reports for the transitions of the $P$ polymorph. The anomaly observed by dielectric measurements in the vicinity of $150°C$ was frequency dependent and could be related to the dynamics of the ferroelectric nanoregions. The phase transitions of the as-synthesized $NaNbO3$ powder were investigated using differential scanning calorimetry and high temperature x-ray diffraction. The results show the existence of the $Q$ polymorph at room temperature, not previously reported for the powder, which undergoes a transition to the $R$ polymorph upon heating through a temperature region between 265 and $326.5°C$⁠. This transition is mainly related to the displacement of Na into a more symmetric position and a minor change in the tilting system. The structures at room temperature, 250, 300, and $420°C$ were refined by the Rietveld method and the evolution of the tilting system of the octahedral network and cationic displacement are reported.