Comparative Raman study of the precursor order in KLT and KTN

A recent Raman study of the hard optic modes TO2 and TO3 in K1-xLixTaO3 (KLT) and KTa1-xNbxO3 (KTN) has confirmed the existence, above the transition, of precursor order in the form of polar nanoregions. These dynamical regions give rise to otherwise forbidden first-order Raman scattering. The primary difference between KLT and KTN is found in the temperature dependence of this impurity induced scattering. With a higher relaxation frequency of the bare dipoles in KTN, the nanoregions appear much more dynamic in KTN than they do in KLT. Consequently the TO2 impurity induced scattering appears at lower temperature and increases more rapidly. The difference between KTN and KLT is also revealed by a detail analysis of the TO2 lineshape above the transition, which is seen to be composed of a dynamic and a quasi-static component. The relative contributions of these two components are compared in the two systems. A microscopic theory of the lineshape is also presented.

Keywords:

• Raman scattering
• precursor order
• polar phonon
• collective dynamics