Raman spectroscopy of Lithium modified Sodium Niobate at elevated temperature

We have carried out high temperature Raman spectroscopy studies on a technologically important material Li0.12Na0.88NbO3 (LNN12) in a wide temperature range from 298 K to 1073 K. The variations of Raman mode frequencies and full width at half maximum point towards various structural transitions in this temperature range. A new mode which appears around 650 cm(-1) at 473 K has been found to have anomalous temperature dependence. Furthermore, the symmetric stretching and the asymmetric stretching modes have opposite temperature dependence in some temperature ranges. At the highest temperature 1073 K (in cubic phase), there are two broad bands centered around 200 cm(-1) and 600 cm(-1). The transition temperatures obtained by Raman scattering study are found to be consistent with those obtained by powder diffraction techniques. Softening of stretching modes across transitions is correlated with the symmetrization of NbO6 octahedra in the compound.