INELASTIC-NEUTRON-SCATTERING STUDY OF THE CUBIC-TO-TETRAGONAL TRANSITION IN K0.965LI0.035TAO3

For over ten years the existence of a ferroelectric phase transition i n K1-xLixTaO3 has been the object of a strong controversy despite earl y Raman results which revealed a splitting of the soft TO1 mode at low temperature. We report here the results of an inelastic-neutron-scatt ering study of a 3.5 at. % Li crystal which show, for q not equal O, a corresponding splitting of the TA branch between a-polarized and c-po larized phonons and no condensation of the TA mode. The phonon assignm ent is based on measurements made in the presence of a bias electric f ield. These results confirm the existence of a cubic-to-tetragonal tra nsition in K1-xLixTaO3 even for relatively low concentrations. They re veal the order-disorder nature of this transition and the strong coupl ing of c-polarized TA phonons to the spontaneous polarization. A stron g anisotropy of the dispersion surface is also shown to explain the as ymmetric shape of the phonon peaks.