Structural, dielectric, and ferroelectric properties of Na0.5(Bi1-xNdx)0.5TiO3 ceramics for energy storage and electrocaloric applications

Abstract Lead-free relaxor ferroelectric Na0·5(Bi1-xNdx)0.5TiO3 (xNdNBT, x = 0–30%) ceramics were synthesized using the conventional solid-state method. Detailed Rietveld refinement revealed transitions from rhombohedral (x = 0, 5%), to a coexistence of R3c and Pnma symmetries (x = 10, 15%), followed by a tetragonal P4/mmm phase for the high Nd-concentrations (x = 25, 30%). High-temperature dielectric stability was found over a large temperature range (for instance ±15% from -90 °C to 405 °C for x = 30%) with very low dielectric losses, especially at high temperatures. The substitution of Bi3+ by Nd3+ results in a perturbation of the ferroelectric order and favors short-range polar regions in the system. From the ferroelectric measurements, we demonstrated the stabilization of a coexistence of non-ergodic and ergodic relaxor state or ergodic relaxor state at specific doping concentrations. Besides, high-energy storage properties were obtained at high temperatures, with Wrec > 1.15 J/cm3 in the temperature range 140 °C–200 °C for the 5NdNBT and a moderate Wrec between 0.5 and 0.77 J/cm3 in a large temperature range from 25 °C to 200 °C and with high efficiency >70% for 10NdNBT composition. In addition, a high electrocaloric response (ΔT = 1.8 K at 95 kV/cm) was found using the indirect method for the 5NdNBT composition, with great temperature stability below 20% over a broad temperature range (80–140 °C) and with a high ΔT/ΔE > 0.19 K m/V. The obtained electrocaloric response was related to the thermal evolution of the polar regions in mixed ergodic and non-ergodic relaxor states under an applied electric field.