Temperature dependence of the Raman modes inLiNbO3and mechanism of the phase transition

We have performed Raman-scattering measurements on ${\mathrm{LiNbO}}_3$ as a function of temperature. In the configuration $\mathrm{}\left(\mathrm{ZZ}\right)\mathrm{}$ both a soft phonon $A_1$ (TO) and a quasielastic scattering have been observed and carefully analyzed within a model including a decoupled damped oscillator and a Debye relaxation mode. Good agreement was achieved in the dielectric constant between the calculated values deduced from Raman studies and the experimental data reported by Tomeno and Matsumara. This agreement proves the validity of our fitted phonon parameters and supports the mainly order-disorder character for the ferroelectric-paraelectric phase transition of ${\mathrm{LiNbO}}_3.$