Effect of structural transition on magnetic and optical properties of Ca and Ti co-substituted BiFeO3 ceramics

Bi1−xCaxFe1−xTixO3 ceramics with x = 0.0, 0.05, 0.075, 0.10, 0.125, 0.15 and 0.2 nanoparticles were prepared by the tartaric acid modified sol-gel technique. The co-substitution of Ca and Ti in certain proportion controls the formation of secondary phases. Structural phase analysis by XRD suggested that pure bismuth ferrite stabilized in rhombohedral crystal symmetry (space group R3c) and orthorhombic (Pbnm) phase fraction was observed in co-substituted samples which increase with the increase in substitution percentage. The changes in the phonon frequencies (A1) and line widths in Raman spectra reveal the lattice distortion in co-substituted samples which are in agreement with our XRD analysis. The improved magnetization and coercive field in co-substituted samples occurs due to the suppression of cycloid spin structure which could be explained in terms of field induced spin reorientation and weak ferromagnetism. The Ti4+ substitution at Fe site in BiFeO3 significantly reduced the oxygen vacancies and hence the associated leakage current, which leads to the increase of dielectric constant as well as frequency independent region for ∊r and tan δ (maximum in Bi0.85Ca0.15Fe0.85Ti0.15O3). The significant decrease in dielectric loss with the increase in substitution percentage leads to the decrease in the room temperature bulk conductivity and thus enhanced resistivity in co-substituted samples. The band gap energy of substituted samples was found to be red shifted with the increase in the substitution percentage due to increased lattice strain.