Raman study of the transition dynamics in KLT and KTN

Abstract

A Raman scattering study of the hard TO₂ optic mode in KTaO₃:Li (KLT) and KTaO₃:Nb (KTN) has revealed the existence of dynamical polar nanoregions. Associated with these precursor effects, the TO₂ lineshape exhibits noticeable changes. In KLT two such changes have been observed, the first one at about 200K and the second one through the transition. The behavior in KTN is different in that only one change in the linewidth (which occurs at the transition) can be resolved. A model explaining this behavior is proposed. In KLT, the high temperature change in linewidth can be attributed to the increase in the lifetime of the polar nanoregions, such that the dynamical or homogeneous broadening becomes less than the static or inhomogeneous broadening. The second (lower temperature) decrease in the linewidth therefore results from the rapid increase in the correlation length at the transition. In KLT, below the transition, the width continues to slowly decrease which is indicative of a more progressively increasing correlation length. In the case of KTN the simultaneous increase in the lifetime and in the correlation length of the polar fluctuations is due to the much higher reorientational frequency of Nb compared to that of Li in KLT.