Phase transition of LixNa1-xNbO3 studied by Raman scattering method

In this study, we use X-ray diffraction and Raman scattering techniques to study the structure and phase transition of lithium sodium niobate crystal. Raman scattering experiments on these ceramics have been performed in the temperature range from 300 to 77 K. All the spectral lines in the Raman spectra can be assigned as the transitional modes of cations Li+ and Na+ and the internal modes of NbO6 octahedron. From the evolution of the spectral line in the Raman spectra at various temperatures, we conclude that phase transition of all three crystals of x = 0.1, 0.12, 0.15 occurs over a temperature range from 200 to 180 K on the cooling process and at 280 K in the heating process. The phase transition of other compositions is also similar to the pure sodium niobate, but the transition temperatures are different. The lattice structure at room temperature was also studied by X-ray diffraction. The study shows that the crystal demonstrates a single phase of orthorhombic structure for x less than or equal to 0.15, two phases above x = 0.2, and then changes to LiNbO3 structure until x = 0.9. These results agree well with the results from x-dependent Raman scattering experiments. From temperature-dependent Raman scattering study, we have shown that the Raman spectra of LixNa1-xNbO3 exhibit a phase transition below room temperature that is similar to that observed in NaNbO3. The transitional motion of the Na+ cation may be involved in the structural phase transition of lithium sodium niobate. (C) 1999 Elsevier Science Ltd. All rights reserved.