Optoelectronic Properties And Structural Evolution Of Na_0.5Bi_2.5Nb₂O₉ Ceramics

The remarkable character of relatively high Curie temperature（T_c）,low dielectric dissipation,strong anisotropic electromechanical properties,and high mechanical quality factors make the bismuth layer-structured ferroelectrics available for high temperature sensor applications.However,the low residual polarization,the high coercive fields,the low piezoelectric activity and piezoelectric activity at elevated temperature of these materials limit the potential applications.Therefore,to improve the performance of these materials has always been a hot spot of research.Doping is one of the most commonly used method to improve performance.At present,the main work is based on the electrical measurement,such as ferroelectric and dielectric properties.It is lack of analysis of microstructure.Therefore,the optical properties of Na_0.5Bi_2.5Nb₂O₉ ceramics were studied by optical means to analyze the microstructure.Moreover,further analysis was carried out by the first principle.The main optical methods used were infrared reflectance spectroscopy,Raman scattering and spectroscopic ellipsometry.The main work and innovation of this article are as follows.（1）The effects of W doping and temperature on the structure of Na0.5Bi2.5Nb2O9 ceramics were analyzed by X-ray diffraction,infrared reflectance（IR）,Raman scattering and spectroscopic ellipsometry（SE）.The analysis of XRD,IR and SE at room temperature indicated that the lattice symmetry had been improved and the optical band gap increased with increasing W content,which could be attributed to the smaller ionic radius and higher valance state of W⁶⁺than Nb⁵⁺.The introduction of W⁶⁺ions not only affects the vibration of B-site cation,but also affects the cation vacancy in the（Bi₂O₂）²⁺layer.Moreover,IR（6-300K）,Raman scattering（77–800 K）and SE（200–800 K）systemically explored the temperature dependent lattice dynamics and electronic properties,respectively.The anomalous temperature dependence of phonon frequencies and electronic transitions around 600 K could be ascribed to the existence of an intermediate phase.This was the first time to find the intermediate phase transition of Na_0.5Bi_2.5Nb₂O₉ ceramics between ferroelectric and paraelectric phases.（2）The effects of BaTiO3 doping and temperature on the structure of Na0.5Bi2.5Nb2O9 ceramics were analyzed by X-ray diffraction,IR,Raman scattering and SE.The spectral analysis at room temperature of（1-x）Na_0.5Bi_2.5Nb₂O₉-xBaTiO₃（0≤x≥0.08）indicated that the doping of BaTiO₃ increased the lattice constant and the tolerance factor t.The increase of t also shows the improvement of the structure symmetry of perovskite-like layer.From the IR and Raman scattering,it was found that the doping of A-and B-site cations led to a larger stacking fault between（Bi₂O₂）²⁺layer and perovskite layer,which reduced the structure symmetry.The analysis of spectroscopic ellipsometry spectra shown that the introduction of BaTiO₃ led to a certain change in the optical band gap.The intermediate phase was also found by temperature dependent Raman scattering（77–800 K）and SE（200–800 K）.（3）Structure and band analysis of Na0.5Bi2.5Nb2O9 by the first principles calculation in the ferroelectric phase,intermediate phase and paraelectric phase.We chose space group Amam as the reference structure by referring to Sr Bi₂Nb₂O₉.In the paraelectric Amam phase,the distortion of NbO₆ octahedra is decreasing.Moreover,there are no tilt and rotations of the NbO₆ octahedra in paraelectric phase structure,ensuring the structural symmetry of the body-centered tetragonal system.In addition,the variation trends of optical band gap between different phases are in good agreement with the SE experimental results.It indicates that the variations of electronic transitions which is applied to judge the phase transformations in ferroelectric materials.