Lead-free antiferroelectric: xCaZrO3-(1 − x)NaNbO3 system (0 ≤ x ≤ 0.10)

Abstract

This study demonstrates that antiferroelectricity can be stabilized in NaNbO₃ (NN) based ceramics by lowering the tolerance factor. Through consideration of the crystal chemistry via the Goldschmidt tolerance factor and polarizability, we show that simultaneous substitution of Zr⁴⁺ and Ca²⁺ ions in the Nb and Na sites, respectively, lowers the polarizability and tolerance factor of the (Na_1−xCa_x)(Nb_1−xZr_x)O₃ (CZNN100x) solid solution, while maintaining charge neutrality. Structural investigations using both X-ray diffraction and transmission electron microscopy (TEM) indicated an enhancement of antiferroelectric (AFE) superlattice peaks with CaZrO₃ substitution. The TEM domain analysis revealed that only AFE domains existed in the CZNN4 and CZNN5 ceramics; in contrast, normal NN ceramics displayed coexistence of AFE and ferroelectric (FE) domains at room temperature. The CZNN100x (0.02 ≤ x ≤ 0.05) ceramics showed double polarization hysteresis loops, characteristic of reversible AFE↔FE phase transition switching. The field-induced polarization decreased drastically with increasing substitution, an effect of the decreases in tolerance factor. In addition, the AFE switching field was increased by the chemical substitution. First principles calculations are performed to obtain insights into the relative stability and coexistence of the AFE and FE phases in single domains. The large decrease of polarization in the CZNN system is explained by a modification of the relative stability of the relevant structures, which favours nonpolar-to-polar AFE transitions over polar-to-polar FE domain switching.