High-pressure Raman study and pressure-induced phase transitions of sodiumniobate NaNbO3

Raman spectroscopic studies have been carried out on antiferroelectric sodi um niobate NaNbO3 under pressures up to 22 GPa, The Raman spectra show a ri ch variety of changes with pressure and evidence for several phase transiti ons were observed, A first-order phase transition was found at 7.0 GPa, A l ow wavenumber mode at 42 cm(-1) appears at 6.0 GPa as a precursor, which so ftens at higher pressure. This band is obscured above 7.0 GPa after the fir st order phase transition. Another possible phase transition is proposed at 12 GPa, indicated by the sign change of the gradient of wavenumber-pressur e plots for several modes and the disappearance of the Raman band for the s trong vl 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 ferr oelectric and the phase above 12 GPa is probably the paraelectric phase. Co pyright (C) 2000 John Wiley & Sons, Ltd.