Strong anharmonicity and lattice dynamics in LiTaO3 by Raman spectroscopy

The phonon-dispersion relations of LiTaO3 crystal are calculated using the first-principle calculations approach described earlier. We found that the TO1 (206 cm-1) and the TO2 (256 cm-1) are the z-vibration of Li ions and other ions, respectively, while the z-displacements are negligible in the TO3 (357 cm-1) and the TO4 (600 cm-1) modes using this calculation. The first-order Raman spectra in the LiTaO3 were measured in the temperature range 23-896K for rhombohedral phase. In the experiment the TO2 mode softens more rapidly than the TO1 mode and then pushes it down to lower frequencies. The temperature dependence of linewidth of four TO modes was analyzed using TDOS due to cubic term based on the first-principle calculations. The calculated results reproduce the observed ones in this temperature range. We found that the ionic motions associated with the TO1 and the TO2 modes are transferred to each other, and in the TO4 mode, torsion of individual octahedral take place, giving a strong anharmonic contribution.