Structural and vibrational properties of (Bi1−xLax)FeO3 and (Bi1−yBay)(Fe1−yTiy)O3 multiferroic ceramics investigated by Raman scattering

La-modified Bi1−xLaxFeO3 (x = 0.15-0.40) and BaTiO3-modified (Bi1−yBay)(Fe1−yTiy)O3 (y = 0.10−0.33) multiferroic ceramics were synthesized via solid-state reactions, and their structural and vibrational properties were investigated by X-ray diffraction (XRD) and Raman spectroscopy. The XRD patterns reveal that the La-modified Bi1−xLaxFeO3 system exhibits an rhombohedral - orthorhombic structural transition at x = 0.40. The degree of rhombohedral distortion in the Bi1−xLaxFeO3 ceramics was decreased with increasing the La content, resulting in the disappearance of some Raman active modes (e.g. A1-4, E(TO1), E(TO2), E(TO6)). Similarly, in the BaTiO3-modified (Bi1−yBay)(Fe1−yTiy)O3 system, a composition-driven structural transition from rhombohedral (R3c) phase to cubic phase appeared at y = 0.30, indicating a morphotropic phase boundary between the rhombohedral and cubic phases. It is found that the Raman mode frequencies of the A1-1, A1-2, A1-3 and E(TO3) modes are slightly dependence of either the La content in Bi1−xLaxFeO3 system or the BaTiO3 content in (Bi1−yBay)(Fe1−yTiy)O3 system, whereas the frequencies of the E(TO8) and E(TO9) Raman active modes are significantly dependent upon the modified compositions. Such compositional dependence of the Raman active mode frequencies are discussed and interpreted based on a spring oscillator model in combination with the structural phase transition, the degree of rhombohedral distortion, and the B-O bond length in the BO6 octahedron. The present results are useful for understanding the correlation between the structural and physical properties of the La-modified Bi1−xLaxFeO3 and the BaTiO3-modified (Bi1−yBay)(Fe1−yTiy)O3 ceramics.