Microstructure,Electrical Behavior And First-principles Calculations Of Na_0.5Bi_0.5TiO₃ Ferroelectric Ceramics

Perovskite ferroelectrics have special crystal symmetry,which can produce spontaneous polarization and effectively regulate ion displacement under the action of electric field.Sodium bismuth titanate(Na_0.5Bi_0.5TiO₃,NBT)is a kind of ABO₃ type lead-free perovskite ferroelectric with a high spontaneous polarization value,which can replace the existing lead-based ferroelectric materials.So far,the researches on its structure and properties have focused on two main problems:?1?the distribution of A cation of NBT is still controversial due to the unequivalent valence of Na and Bi elements;?2?good ferroelectrics depend on insulation.Since Bi element is easy to volatilize during heat treatment,leakage caused by defects worsens the electrical performance of NBT.This project combines the experimental characterization and first-principles calculations to deeply explore the mechanism of the A-site distribution and its effects on the intrinsic structure and electrical properties of NBT.XRD,TEM,Raman spectrum and dielectric measurements were applied to study the nanoscale regions in NBT and its influence on relaxation behavior,and the local structure?nano-regions?in NBT was detected at room temperature.The chemical composition in the nano-regions was detected by TEM as rich in Bi.The Bi-rich nano-regions induce the relaxation behaviors of NBT at high frequencies.In order to solve the difficult polarization and improve the electrical properties of NBT,the project also prepared the lead-free ferroelectric NBT,NBT-6BT and Nb-NBBT6ceramics via a multi-ionic sole-gel method process.The symmetry of NBT was mainly affected by the A-site ion,but less by the B-site ion.The doping ions can enhance the relaxation of NBT,and obtain the stable permittivity in relatively high temperature and wide temperature range.The polarization was easier to switch in the doped ferroelectric ceramics.The competitive energy storage density of Nb-NBBT6 ceramics indicated the potential energy storage applications at both ambient and high temperature.First-principles calculations showed that NBT had fluctuation of A-site cation.Considering the extreme case,Na⁺TiO₃ unit cell had the highest energy and failed to converge during the calculations,indicating the unstable crystal structure of Na⁺TiO₃.Bi³⁺TiO₃ unit cell possessed the lowest energy,which verified that Bi-riched nano-regions can be easily observed by NBT experimentally.VCA and supercell methods confirmed the increased lattice constant of NBT by Bi enrichment.The enriched Bi atoms shifted the electron density of Bi-O bonding to the low energy region,indicating the strong and stable Bi-O bonding in Bi-rich nano-regions.Moreover,the influence of the strong Bi-O bonding was localized,which verified the nano-scale of Bi-rich nano-regions.The calculated Born effective charges of NBT illustrated the strong polarization of electrons in Bi-O and Ti-O bondings.The spontaneous polarization?P_s?of rhombohedral NBT unit cell was estimated as 0.78 C/m²?77.54?C/cm²?,indicating the favourable ferroelectricity of NBT as well as its tremendous potential in the modifications and applications.