The antiferroelectric to ferroelectric phase transition in NaNbO3 was investigated by Raman scattering between 295 and 10 K. The transition occurs at 185 K on cooling and the antiferroelectric and ferroelectric phases coexist at low temperature. The transition occurs at 210 K on heating and the relatively large hysteresis is attributed to the difficulty in reversing the spontaneous polarization direction at the transition. The results agree well with low-temperature x-ray data which show that both phases coexist if the crystal samples are twinned. The transition is not considered to be of the displacive type, although two modes at 115 and 121 cm-1 were observed to soften at low temperature.
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Shen et al. (1998) studied this question.