Hard-mode Raman spectroscopy and structural phase transition in the relaxor ferroelectric lead scandium tantalate, Pb(Sc_0.5Ta_0.5)O₃

The phase transition in the relaxor ferroelectric lead scandium tantalate, Pb(Sc_0.5Ta_0.5)O₃, is investigated in both ordered and partly disordered (Sc, Ta disorder) modifications through the technique of hard-mode Raman spectroscopy. The vibrational modes for ordered para phase, ordered ferroelectric phase, disordered para phase and disordered ferroelectric phase are assigned to different representations of the respective groups (Fm3m (Z=2), R3m (Z=2), Pm3m (Z=1) and R3m (Z=1)) using the usual group-theoretical methods. The temperature evolution of the strong Raman band at 61cm^-1 (hard mode) is investigated from 102 K to 873 K, for both phase transitions, namely ordered para to ordered ferro and disordered para to disordered ferro phases. The temperature variations of integrated intensity, full width at half maximum and square of the frequency of the 61cm^-1 band are discussed in the light of phase transition mechanisms involved in relaxor ferroelectrics for both cases.