Raman scattering from La-substituted BiFeO3–PbTiO3

Polycrystalline samples of pure BiFeO3–PbTiO3 (BF–PT) and those upon substituting Bi by La up to 50% (x≤0.5) were synthesized using a solid-state reaction method. A composition-driven tetragonal–cubic transition, identified from X-ray diffraction, was found to occur at x=0.4. We report the phonon spectra of pure and La-substituted BF–PT and its assignment. From the behavior of total Raman intensity, the origin of the symmetry-forbidden Raman spectra is identified to lie in substitutional disorder. The dependence of splitting of A1–E modes and that of the LO–TO modes on composition are obtained and their consequences on ionicity are discussed.