Far-infrared soft-mode behavior in PbSc1∕2Ta1∕2O3 thin films

Temperature dependences of the optic phonons in PbSc1∕2Ta1∕2O3 (PST) sol-gel films deposited on sapphire substrates were studied by means of Fourier transform far-infrared transmission spectroscopy in the temperature range of 20–900K. Four films displaying different B-site order with both ferroelectric and relaxor behavior were studied. In all cases the TO mode near 80cm−1 at 10K softens on heating to ≈45cm−1 following the Cochran law with extrapolated critical temperature near 700K which is 400K above the temperature of dielectric maximum, Tm. Above 600K the TO1 mode remains stabile. This mode can be assigned to the A1 component of the ferroelectric soft-mode inside polar clusters which form below the Burns temperature near 700K. In the ordered PST film another mode activates below Tm in the infrared spectra near 60cm−1, also exhibiting an anomalous temperature dependence due to its coupling with the former mode. It is assigned to the A1 component of the F2g Raman active mode. The central mode, which app...