Effect of Tb substitution on structural, optical, electrical and magnetic properties of BiFeO₃

Tb substituted BiFeO₃ [Bi_1−xTb_xFeO₃ (x = 0.05, 0.10, 0.15)] have been synthesized by a low temperature assisted Co-precipitation method. Rietveld-refinement of the X-ray diffraction data reveals a transition from rhombohedral (R3c) to orthorhombic (Pnma) phase, i.e. polar to non-polar phase with Tb substitution. The crystallite sizes of Bi_1−xTb_xFeO₃ (x = 0.05, 0.1 and 0.15) are found to be approximately 30, 21 and 15 nm calculated using Debye–Scherrer equation. From transmission electron microscopy analysis, the particle sizes are found to be between 35–40, 30–35, and 25–30 nm, respectively for Bi_1−xTb_xFeO₃ (x = 0.05, 0.10 and 0.15) samples. UV–Vis diffuse reflectance spectra show a decrease of band gap with increase in Tb concentration. 4A1 and 7E Raman modes have been observed in the range 100–650 cm⁻¹ and two phonon modes centred around 1150–1450 cm⁻¹ have also been observed. The changes in Raman modes such as prominent frequency shift, line broadening and intensity reveals the existence of substitution induced structural changes as supported by the Rietveld refinement. Temperature dependent dielectric measurements on the samples show magnetoelectric coupling in terms of a dielectric anomaly near the Neel temperature (T_N). An enhancement of magnetization with increasing Tb concentration in BFO has been observed from room temperature magnetization studies. The leakage current density is found to be reduced with the increase of Tb concentration. Further, P–E hysteresis loop studies show a decrease of remnant polarization (P_r) with the increase in Tb concentration predicting a transition from ferroelectric (polar) to paraelectric (non-polar) phase as inferred from X-ray diffraction analysis.