Soft Modes and Superlattice Structures in NaNbO₃

From the study of superlattice reflections in X-ray diffraction, phase transitions at 365°C, 480°C, 520°C, 572°C and 643°C in NaNbO ₃ are definitely identified by some distinct changes of the positions and intensities. The anisotropic diffuse X-ray scatterings have been studied with a special respect to the phase transitions. The modes of vibration which are responsible for the diffuse scatterings are found to be \(\varGamma_{25}\) and M ₃ modes at R and M points, respectively. It is found from anomalous increases of the diffuse intensities that the M ₃ mode condenses at 643°C and E _g mode, into which two of the \(\varGamma_{25}\) mode transform, condenses at 572°C. The behaviors of the zone boundary phonons in NaNbO ₃ are compared with those in KMnF ₃ . The correlation of superlattice structures with the instability of the zone boundary phonons is discussed.