Correlation between ionic size and valence state of tetra, penta and hexavalent B-site substitution with solubility limit, phase transformation and multiferroic properties of Bi0.875Eu0.125FeO3

Highlights: • Comparative effect of polyvalent ion substitution in BiFeO₃. • Increased microstrain and induced chemical pressure by the substituent helps in triggering the phase transformation. • Ionic radii mismatch and valance difference of parent and host ion effect the solubility limit of different dopants. • Results has been explained in terms of bond strength differences arising due to inclusion of higher valent dopants. - Abstract: We present detailed comparative study of effect of isovalent i.e. Eu⁺³ substitution at A-site and tetra (Ti⁺⁴, Zr⁺⁴), penta (V⁺⁵) and hexavalent (W⁺⁶) substitutions at B-site in BiFeO₃. Eu⁺³ substitution led to phase transformation and exhibited mixed phases i.e. rhombohedral and orthorhombic, while tetravalent substituents (Ti⁺⁴ and Zr⁺⁴) led to stabilization of cubic phase. In higher valent (i.e. V⁺⁵ and W⁺⁶) cases solubility limit was significantly reduced where orthorhombic phase was observed as in the case of parent compound. Phase transformation as a consequence of increase in microstrain and chemical pressure induced by the substituent has been discussed. Solubility limit of different B-site dopants i.e. Zr, W and V was extracted to 5%, 2% and 2%, respectively. Extra phases in various cases were Bi₂Fe₄O₉, Bi₂₅FeO₄₀, Bi₁₄W₂O₂₇, and Bi₂₃V₄O_44.5 and their fractional amount have been quantified. Ti was substituted up to 15% and has been observed to be completely soluble in the parent compound. Solubility limits depends on ionic radii mismatch and valance difference of Fe⁺³ and dopant, in which valance difference plays more dominant role. Solubility limit and phase transformation has been explained in terms of change in bond strength and tolerance factor induced by incorporation of dopant which depend on its size and valence state. Detail optical, dielectric, ferroelectric, magnetic and transport properties of Eu and Ti co-doped samples and selected low concentration B-site doped compositions (i.e. 2%) have presented and discussed. Two d-d transitions and three charge transfer transitions were observed within UV-VIS range. Both change in cell volume for the same phase and transformation in crystal structure affects the band gap. Increase in room temperature dielectric constant and saturation polarization was also found to increase in case of Eu-Ti co-doped samples with increasing concentration of Ti. Substitution of Eu at A-site and Ti at B-site led to observation of weak ferromagnetism. Effect of extra phases on ferroelectricity and transport properties have also been discussed.