Investigation of the relaxor system Pb(1-x)BaySc(1/2)Nb(1/2)O(3-x+y) by Raman scattering

Lead scandium niobate PbSc1/2Nb1/2O3 undergoes a structural phase transitio n at about 380 K from a cubic phase (of average symmetry Pm (3) over bar m to a slightly distorted polar trigonal phase (R3m). Two order parameters (p olarization and ferroelastic distortion) are involved in this phase transit ion and some anomalies are expected in the temperature evolution of the Ram an spectra of the various compounds belonging to this system (ordered lead scandium niobate, disordered, Ba-doped and Pb-vacancy). The spectra collected between 600 K and room temperature, are analyzed and compared with group-theoretical predictions. The number of observed frequen cies in the high temperature phase of the different compounds is larger tha n the value expected for cubic symmetry phases (Pm (3) over bar m or Fm (3) over bar m either). The implications of substitutional and positional diso rder leading to a local symmetry breaking are discussed.