Effect of k-ion-rich substitution on structural, thermally assisted relaxation processes in Na0.5Bi0.5TiO3 relaxor ferroelectric

Abstract

Utilizing a complex impedance technique, we employed a probing approach to investigate the dielectric relaxation and electrical transport characteristics in lead-free Na0.5-xKxBi0.5TiO3 compositions (where x = 0.30, 0.35, 0.40, and 0.45). X-ray diffraction and Raman analysis confirmed the tetragonal phase of all compositions. This study affirms the presence of mostly bulk resistive (grain) contributions in the materials. The most notable grain resistance (Rg) was found at approximately 0.8 M ohm-m for the composition with x = 0.45 at a temperature of 500 °C. The increase in Rg strongly supports the role of K+1 as a suppressor of grain growth, leading to an increased presence of grain boundaries with an enhancement in intrinsic breakdown strength. This study validated the occurrence of non-Debye-type dielectric relaxation across all examined compounds with Negative Temperature Coefficient of Resistance (NTCR) type behavior. The pivotal role of oxygen vacancies in the conduction mechanism was affirmed through Arrhenius's theory. Particularly, the most elevated Ea values, 1.37 eV, and 1.48 eV, were observed in the case of x = 0.45 (NKBT-45). The similarity between activation energies derived from conduction and relaxation processes supports that the responsible charge carriers governing electrical transport remain consistent. Moreover, evidence of Polar Nanoregions (PNRs) formation is indicated by the separation observed between normalized Z″(f) and M″(f) peaks in NKBT-45. This observation suggests a transition from long-range conduction to localized dipolar relaxation.