Enhanced energy storage properties and antiferroelectric stability of Mn-doped NaNbO₃-CaHfO₃ lead-free ceramics: Regulating phase structure and tolerance factor

NaNbO₃-based ceramics usually show ferroelectric-like P-E loops at room temperature due to the irreversible transformation of the antiferroelectric orthorhombic phase to ferroelectric orthorhombic phase, which is not conducive to energy storage applications. Our previous work found that incorporating CaHfO₃ into NaNbO₃ can stabilize its antiferroelectric phase by reducing the tolerance factor (t), as indicated by the appearance of characteristic double P-E loops. Furthermore, a small amount of MnO₂ addition effectively regulate the phase structure and tolerance factor of 0.94NaNbO₃-0.06CaHfO₃ (0.94NN-0.06CH), which can further improve the stability of antiferroelectricity. The XRD and XPS results reveal that the Mn ions preferentially replace A-sites and then B-sites as increasing MnO₂. The antiferroelectric orthorhombic phase first increases and then decreases, while the t shows the reversed trend, thus an enhanced antiferroelectricity and the energy storage density W_rec of 1.69 J/cm³ at 240 kV/cm are obtained for 0.94NN-0.06CH-0.5%MnO₂(in mass fraction). With the increase of Mn content to 1.0 % from 0.5 %, the efficiency increases to 81 % from 45 %, although the energy storage density decreases to 1.31 J/cm³ due to both increased tolerance factor and non-polar phase.