J. Phys. IV France
Volume 128, September 2005
Page(s) 121 - 126

J. Phys. IV France 128 (2005) 121-126

DOI: 10.1051/jp4:2005128019

Dynamics of polar clusters in relaxor ferroelectrics

S. Kamba1, M. Kempa1, M. Berta1, J. Petzelt1, K. Brinkman2 and N. Setter2

1  Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 18221 Prague 8, Czech Republic
2  Ceramics Laboratory, EPFL, Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland

The paper brings results of far infrared transmission studies of relaxor ferroelectric PbMg1/3Nb2/3O3 and PbSc1/2Ta1/2O3 thin films performed at temperatures between 10 K and 900 K. The spectra clearly show that the lowest-frequency polar optic phonon softens (partially) neither towards the temperature of dielectric-constant maxima, nor towards the freezing temperature, but towards the Burns temperature. This indicates that it represents the ferroelectric soft mode inside the polar clusters. The dielectric relaxation, which expresses the dynamics (flipping and breathing) of polar clusters, appears in THz range below the Burns temperature. This relaxation dramatically slows down to sub-Hertz region on cooling to freezing temperature near 200 K (in PMN) or to T$_{\rm c}$ (in PST). Another heavily damped excitation remains in the spectra below 30 cm $^{\hbox{-}1}$ down to 10 K. This mode was assigned to the E component of the split F $_{1{\rm u}}$ soft mode.

© EDP Sciences 2005