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.