Defect dipole induced improved electrocaloric effect in modified NBT-6BT lead-free ceramics

The Rietveld refinement of the polycrystalline powders of 1% Fe and Mn-doped (Na_0.5Bi_0.5)_0.94Ba_0.06Ti_0.98V_0.02O₃ at the Ti-site confirmed a single rhombohedral (R3c) phase. The bandgap, (E_g) was affected by the anti-phase octahedral tilt angle and the spin-orbit splitting energy of Ti⁴⁺2p_3/2 and Ti⁴⁺2p_1/2 states. The decrease in Bi loss and increase in the binding energy of Ba due to Fe/Mn doping has been correlated to the strengthening of Bi-O and Ba-O bonds which was revealed from the XPS studies thereby further related to the average A-O bond length from structural studies. Hence, a reduction of oxygen vacancy (V_O) for the doped samples has been justified. A significant improvement of the dielectric constant, relaxation time (τ₀), and the decrease in conductivity due to doping was revealed from the frequency-dependent (10 Hz-1 MHz) dielectric measurement study. The conduction and relaxation process are dominated by the short-range movement of defects. The activation energy (E_a ~1 eV) revealed that there is a presence of double-ionized V_Os. The ECE study showed a significant enhancement of the changes in entropy, ΔS, and the adiabatic temperature difference, ΔT, due to doping, with ΔT being highest in the Fe-doped sample. Such improvement of dielectric and ECE properties was confirmed due to the reduction of the mobility of oxygen vacancy because of the formation(Mn_Ti^′′−V_o^••)/(Mn_Ti^′−V_o^••)^• and Fe_Ti^′′−V_o^••/(Fe_Ti^′−V_o^••)^• defect dipoles.