Neutron scattering study of the soft modes in cubic potassium tantalate-niobate

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Abstract
Measurements have been made of the phonon dispersion relation for the transverse optic and acoustic branches of lowest energy in cubic KTN (KTa0.63Nb0.37O3). As in KNbO3 and BaTiO3 one optic branch is soft and overdarnped for all wavevectors q in 〈100〉 directions. The dispersion relation for this branch Js very anisotropic, the energy increasing rapidly with increasing q in 〈110〉 directions for polarization e parallel to 〈110〉 directions. The eigenvectors have been determined at |q| =0.2 a* by measuring the neutron scattering cross section in various Brillouin zones for both the TO and TA branches. The pattern of atomic displacement in the soft mode is found to be primarily of the “Slater” (g1) and “Last” (22) types and does not contain in g3 component. In this respect it resembles the corresponding modes in KTaO3 and SrTiO3 more closely than those in KNbO3 and BaTiO3. At||q|| =0.2 a* the relative atomic displacements in the soft mode are ζK = 3ζTa_Nb,) ζor = ζon = ζom = -2ζTa-Nb The mode contains an acoustic (g0) component which is due, at least in part, to an interaction between the optic and acoustic branches.