Soft-mode behavior and the dipolar glass transition inKTa1−xNbxO3

An inelastic neutron scattering study of ${\mathrm{KTa}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Nb}}_{\mathit{x}}$ ${\mathrm{O}}_3$ (x=0.012) (KTN) shows that there is a marked softening of the zone-center optic mode with decreasing temperature. This mode reaches a nonzero minimum energy of ∼0.9 meV at T=20 K and renormalizes to higher energies as the temperature is further lowered, in agreement with light-scattering results on the same sample. In addition, x-ray diffraction does not reveal any structural phase transition in this temperature range. A recent light-scattering study on a x=0.009 sample suggests that KTN has a dipolar glass phase with a transition temperature of 3 K. The present observation of incomplete softening is similar to the soft-mode behavior characteristic of other mixed systems and provides further evidence of the dipolar glass properties of KTN.