Temperature dependences of the optic phonons in PbSc1/2Ta1/2O3 sol-gel films deposited on sapphire substrates were studied by means of Fourier transform far-infrared transmission spectroscopy in the temperature range 20-900\K. Four films displaying different B-site order with both ferroelectric and relaxor behavior were studied. In all cases the TO mode near 80\icm{} at 10\K{} softens on heating to ≈45\icm{} following the Cochran law with extrapolated critical temperature near 700\K{} (400\K{} above the temperature of dielectric maximum, Tm), but above 600\K{} its frequency remains stabilized. It can be assigned to the A1 component of the ferroelectric soft mode inside polar clusters which form below the Burns temperature near 700\K. In the ordered PST film another mode activates below Tm in infrared spectra near 60\icm exhibiting also anomalous temperature dependence due to its coupling with the former mode. It is assigned to the A1 component of the F2g Raman active mode. Central mode, which appears below the Burns temperature in the THz range, is assigned to the dynamics of polar clusters. It slows down on cooling and vanishes from our spectral range below Tm. Another overdamped excitation assigned to the E component of the soft mode appears near 30\icm{} at low temperatures.
@article{arxiv.cond-mat/0504755,
title = {Far-infrared soft mode behavior in PST thin films},
author = {S. Kamba and M. Berta and M. Kempa and J. Petzelt and K. Brinkman and N. Setter},
journal= {arXiv preprint arXiv:cond-mat/0504755},
year = {2009}
}