Coupled Phonons and Transferred Energies between Raman Active Phonons for LiTaO3 Crystal
Chinese Journal of Physics
|Volume or Term/Year and Month of Publication||
49卷1期（2011 / 02 / 01）
341 - 348
The frequency shift for four TO modes in the LiTaO3 was measured at 23-896K for ferroelectric rhombohedral phase (RH). The TO2 mode (256 cm^(-1)) softens more rapidly than the TO1 (206 cm^(-1)) and then pushes it down to lower frequencies. The temperature dependence of the frequency shift of the four TO modes was analyzed using the first-principles calculations. The calculated results reproduce the observed ones below 350 K. For the frequency shift of the soft TO2 modes, it was explained that the thermal expansion effect Δω0 was partly canceled by the quartic effect Δω(superscript (4a)) with an increase in the temperature up to 350 K. For the TO1 mode, the growth of Δω(superscript (4a)) may cause suppression of softening above 350 K. For the frequency shift of the TO1, TO3, and TO4 modes, it was explained that the thermal expansion effect Δω0 and the cubic effect Δω(superscript (3)) were partly canceled by the quartic effect Δω(superscript (4a)) with an increase in the temperature up to 350 K. As a result, we found that phonon energies with TO1 and TO2 were transferred to each other at the temperature above 350 K. This may be because the ionic motions associated with the TO1 and the TO2 modes are transferred to each other.