INELASTIC NEUTRON-SCATTERING MEASUREMENTS ON THE TRANSVERSE ACOUSTIC AND LOWEST OPTIC MODES IN KTAO3 DOPED WITH LITHIUM

Two crystals of KTaO3:Li with 1% and 5% Li have been investigated by i nelastic neutron scattering. The transverse acoustic and lowest optic modes have been measured in the [100] direction from room temperature down to 10 K. The wavevector dependences of the mode softening, dampin g and intensities are represented. The measured dispersions are analys ed in the framework of an extended shell model assuming an anisotropic non-linear polarizability of the oxygen ion. On the basis of this mod el, structure factors are calculated and compared with measured phonon intensities. Conclusions on the influence of the lithium substitution on the non-linear character of the polarizability are discussed.