| Summary: | 碩士 === 國立成功大學 === 物理學系 === 84 === Raman Scattering experiments on Lio.1Na0.9NbO3, Lio.12Na0.88NbO3 and Li0.15Na0.85NbO3 crystals have been performed in the temperature range from 300 to 77K. All the spectral lines in the Raman spectra can be assigned as the translational modes of cations Li+ and Na+ and the internal modes of NbO3 octahedron proposed by Ross. From the evolution of the spectral line in the Raman spectra at various temperatures, we conclude that phase transition of all three crystals occurs over a temperature range from 200 to 180K in the cooling process and at 280K in the heating process. The thermal hysteresis of the transition is about 80K, which can be attributed to the large structural change required at the transition.
The lattice structure at room temperature were also studied by X-ray diffraction. The study shows that all three crystals have same lattice constants indicating that crystals LixNa1-xNbO3 with X=0.1, 0.12, 0.15 have same crystal structure. This conclusion is supported by the fact that all crystals have same phase transition temperatures from the Raman scattering studies.
Previous studies have shown that crystals LixNa1-xNbO3 have orthorhombic symmetry for X up to 0.12. Our results reduce the X value to 0.1. For crystals with X≧0.1, the crystals lattice have tetragonal symmetry.
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