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. - Graphical abstract: Raman spectrum of the cubic phase (x≥0.4) of La-substituted BF-PT [0.5(Bi1-xLaxFeO3)0.5(PbTiO3)] consists of 7 modes, while group theory does not predict any Raman active phonon. Highlights: → Composition-driven tetragonal-cubic transition in La-substituted BF-PT. → Phonon spectra in the tetragonal phase assigned. → A1-E anisotropy splitting of phonons decreases upon La-substitution. → Increase in LO-TO splitting at larger La composition due to higher ionicity.