Size and Lone Pair Effects on the Multiferroic Properties of Bi0.75A0.25FeO3−δ (A = Sr, Pb, and Ba) Ceramics


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The work presents a comparative study of the effects of divalent Ba, Sr, and Pb substituents on the multiferroic properties of BiFeO3. The multiferroic properties of Bi0.75A0.25FeO3 (A = Sr, Pb, Ba) solid solution have been explained taking into account the effects of size differences and electronic configuration differences between the host element (Bi) and the substituent. X-ray diffraction studies revealed that Sr and Pb substitution at Bi-site transforms the rhombohedral phase (R3c) to cubic phase (Pm3m), whereas the Ba-substituted sample exhibited the presence of both rhombohedral and cubic phases (R3c + Pm3m). Electronic structure studies through XPS revealed that charge imbalance induced by divalent substitution was being compensated by the formation of oxygen vacancies, while the Fe ions exist in Fe2+ and Fe3+ states. Replacement of volatile Bi by Sr, Pb, and Ba reduces the concentration of oxygen vacancies (VO2+) and helps to improve the dielectric properties. Strong magnetization enhancement was observed in the substituted compositions and was seen to be consistent with the suppression of cycloid spin structure due to structural transformation as well as possible changes in FeO local environment leading to local lattice distortion effects. Furthermore, the observed decrease in the values of magnetic coercivity at low temperature in all the substituted samples is explained in terms of reduced effective single ion anisotropy, originating in the magnetoelectric coupling and being a particularly stronger effect in the case of the lone pair dopant Pb, consistent with theoretical predictions. The lone pair substituent Pb leads to the largest dielectric constant, enhanced magnetization, and large effects on the low-temperature hysteresis.