Structural and magnetic properties of isovalently substituted multiferroic BiFeO3: Insights from Raman spectroscopy

Raman spectra, supplemented by powder x-ray diffraction and magnetization data of isovalently A-and B-site substituted BiFeO3 in the Bi1-xLaxFeO3 (0 <= x <= 1), Bi1-xTbxFeO3 (0 <= x <= 0.2), and Bi0.9Sm0.1Fe1-xMnxO3 (0 <= x <= 0.3) series, are presented. A good agreement between the structural transitions observed by x-ray diffraction and the vibrational modes observed in the Raman spectra is found over the whole substitutional ranges, and in particular we find spectroscopic signatures of a PbZrO3-type structure for Bi0.8La0.2FeO3. Mode assignments in the substituted materials are made based on Raman spectra of the end-members BiFeO3 and LaFeO3. Moreover, by comparing spectra from all samples with R3c structure, the phonon assignment in BiFeO3 is revisited. A close connection between the degree of octahedral tilt and the Raman shift of the A(1) oxygen a(-)a(-)a(-) tilt mode is established. An explanation for the strong second-order scattering observed in Bi1-xLaxFeO3 and Bi1-xTbxFeO3 is suggested, including the assignment of the previously mysterious BiFeO3 mode at 620 cm(-1). Finally, the magnetization data indicates a transition from a cycloidal modulated state towards a canted antiferromagnet with increasing A-site substitution, while Bi0.9Sm0.1Fe1-xMnxO3 with x = 0 and 0.15 exhibit an anomalous closing of the hysteresis loop at low temperatures. For low A-site substitution levels (x <= 0.1) the decreasing Raman intensity of the Fe derived modes correlates with the partial destruction of the spin cycloid as the substitution level increases.