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 20-900 K. Four films displaying different B-site order with both ferroelectric and relaxor behavior were studied. In all cases the TO mode near 80 cm(-1) at 10 K softens on heating to approximate to 45 cm(-1) following the Cochran law with extrapolated critical temperature near 700 K which is 400 K above the temperature of dielectric maximum, T-m. Above 600 K the TO1 mode remains stabile. This mode 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 the infrared spectra near 60 cm(-1), also exhibiting an 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. The central mode, which appears below the Burns temperature in the terahertz range, is assigned to the dynamics of polar clusters. It slows down on cooling and vanishes from our spectral range below T-m. Another overdamped excitation assigned to the E component of the soft mode appears near 30 cm(-1) at low temperature. (c) 2005 American Institute of Physics.