COMPOSITION VARIATION OF OPTICAL PHONON DAMPING IN LITHIUM-NIOBATE CRYSTALS

Lattice vibrations have been studied by Raman scattering on stoichiometric, congruent and 5 mol% MgO doped lithium niobate crystals. The mode frequency of the optical phonon shows no marked composition variations, while the damping shows considerable composition dependence. The damping due to the scattering of phonons at defects was determined from the temperature dependence of the damping of A1(z) modes, by the subtraction of the scattering due to the third- and fourth-order anharmonicity. The type of defects which scatter optical phonons was discussed in relation to the Li and Nb site vacancy models. The results strongly support the Li site vacancy model and suggest that the doped Mg atoms occupy the Li site.