Detection of Two Phases Coexisting at the Phase Transition Temperature in LiNbO3 single crystal by using Raman Spectroscopy

Low temperature Raman spectroscopic study has been performed on a Zcut LiNbO3 single crystal in the case of Z(xy)Z configuration. Low temperature studies were performed to be far from the critical temperature, which is known to be at 1165o C in Lithium Niobate, looking for possible anomalies or precursors of the phase transition. Room temperature Raman spectra revealed the presence of the E[TO] modes only, as predicted by the Raman selection rules and group theory for the Z(xy)Z configuration. Comparison of the Raman spectra at room temperature and lower temperatures shows a strong intensity attenuation and progressive reduction in the intensity of the Raman peaks, in addition to the appearance of the A1[TO] modes at 279, 336 and 629 cm-1, which are known to be forbidden modes of the Z(xy)Z configuration. A1[TO] modes resulted from the splitting of 581, 332 and 269 cm-1 vibrational modes into two branches below -100o C. Since the original 581 cm-1 band corresponds to a totally symmetric NbO6 stretching mode, this splitting was interpreted as due to a distortion in the NbO6 structure leading to two distinct sites for the NbO6 unit. This effect is explained as a result of inharmonic vibrations (quasisoft-mode) and the two-phonon state. The temperature dependence of the band parameters showed an inversion point at -100°C, and discontinuity near +150o C. These results indicate that a phase transition occurs at -100o C on cooling and near +150o C on heating, indicating that an abnormality in the structure of LiNbO3 is taking place.