Detection of Two Phases Coexisting at the Phase Transition Temperature in LiNbO3 single crystal by using Raman Spectroscopy | ||||
| Egyptian Journal of Solids | ||||
| Article 4, Volume 31, Issue 2, December 2008, Page 175-184 PDF (266.8 K) | ||||
| Document Type: Original Article | ||||
| DOI: 10.21608/ejs.2008.148804 | ||||
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| Abstract | ||||
| 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. | ||||
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