Far-infrared soft mode behavior in PST thin films

TL;DR

This study investigates the temperature-dependent behavior of optic phonons in PST thin films, revealing soft mode dynamics, coupling effects, and polar cluster activity across a broad temperature range, contributing to understanding relaxor ferroelectric properties.

Contribution

It provides detailed insights into the soft mode behavior and phonon coupling in PST thin films with different B-site order, highlighting their relation to ferroelectric and relaxor characteristics.

Findings

Soft mode near 80 cm^-1 softens with temperature, following Cochran law.

An additional mode near 60 cm^-1 activates below T_m, showing anomalous temperature dependence.

Polar cluster dynamics are evidenced by the central mode in the THz range.

Abstract

Temperature dependences of the optic phonons in $\rm PbSc_{1/2}Ta_{1/2}O_3$ 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 $\approx$45\icm{} following the Cochran law with extrapolated critical temperature near 700\K{} (400\K{} above the temperature of dielectric maximum, $T_{\rm m}$), but above 600\K{} its frequency remains stabilized. It can be assigned to the A$_{1}$ 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 T$_{m}$ in infrared spectra near 60\icm exhibiting also anomalous temperature dependence due to its coupling with the former mode. It is assigned to the A$_{1}$ component of the F$_{2g}$ 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 $T_{\rm m}$. Another overdamped excitation assigned to the E component of the soft mode appears near 30\icm{} at low temperatures.