EN
The ordered PbSc_{1/2}Nb_{1/2}O_3 ceramics were studied by THz transmission spectroscopy in the temperature range of 80-300 K. Below ferroelectric phase transition temperature the strength of central mode gradually decreases and gives evidence for a mixed displacive and order-disorder character of the transitions. Ferroelectric phase transition is connected with an abrupt freezing and rise of polar nanoregions into ferroelectric domains.