  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.
