Asymmetric Raman lines caused by an anharmonic lattice potential in lithium niobate

Spontaneous Raman scattering experiments were performed in stoichiometric and congruent ${\mathrm{LiNbO}}_3$, MgO:${\mathrm{LiNbO}}_3$, and ZnO:${\mathrm{LiNbO}}_3$ in the temperature range from 20 to 500 K. The temperature dependent broadening and asymmetry of the Raman lines of the optical phonons with ${\mathrm{A}}_1$ symmetry is interpreted as the superposition of a series of Raman lines caused by transitions between higher levels in the anharmonic potential for the ionic motion and the decay of the optical phonon into two acoustic phonons. The Raman linewidths in the various doped and undoped ${\mathrm{LiNbO}}_3$ crystals differ by a temperature independent scattering term.