High-pressure Raman study and pressure-reduced phase transitions of sodium niobate NaNbO₃

Raman spectroscopic studies have been carried out on antiferroelectric sodium niobate NaNbO₃ under pressures up to 22 GPa. The Raman spectra show a rich variety of changes with pressure and evidence for several phase transitions were observed. A first-order phase transition was found at 7.0 GPa. A low wavenumber mode at 42 cm^-1 appears at 6.0 GPa as a precursor, which softens at higher pressure. This band is obscured above 7.0 GPa after the first order phase transition. Another possible phase transition is proposed at 12 GPa, indicated by the sign change of the gradient of wavenumber-pressure plots for several modes and the disappearance of the Raman band for the strong υ₁ mode. The spectral changes above 6.0 GPa closely resemble those of KNbO₃, and the results are interpreted in comparison with LiNbO₃ and KNbO₃. The high-pressure phase after the first transition at 7.0 GPa may be ferroelectric and the phase above 12 GPa is probably the paraelectric phase. Copyright (C) 2000 John Wiley and Sons, Ltd.