Effects of La doping on the cubic–tetragonal phase transition and short-range ordering in PbTiO3

Effects of cation-site vacancies on the characteristics of the cubic–tetragonal displacive transitions in La-modified ${\mathrm{PbTiO}}_{\mathrm{3}}$ (PLT) were studied using in situ XRD measurements and Raman scattering. For this purpose, two distinct types of PLT series having cation-site vacancies either at the A site (PLT-A) or at the B site (PLT-B) were separately prepared. The PLT-A series exhibited normal ferroelectricity and underwent the cubic-to-tetragonal structural transition for La substitution up to 30 at. %. Contrary to this, the PLT-B became a typical relaxor with the absence of the macroscopic cubic–tetragonal transition. However, for both types of PLTs, Raman-forbidden bands appeared in the high-temperature cubic-phase field at 30 at. % La modification. These observations were attributed to the formation of short-ranged clusters with tetragonal symmetry in a globally cubic matrix and to the reduced dipolar interaction between the clusters in the PLT-B, as compared with the PLT-A.