Structural Distortion And Optoelectronic Properties Of W-doped CaBi₂Nb₂O₉ Ceramics

High-temperature sensing technology is widely used in chemical and material processing, automotive, aerospace as well as power-generating industries. As the raw material of these sensors, there are so many different types of piezoelectric material to choose from. But when the required operating temperature reaches about 400℃ or higher, the choice of materials for high-temperature piezoelectric devices is limited. There are some single crystals such as LiNbO₃、Si₂Nb₂O₇、La₃Ga₅SiO₁₄ and A₂Ti₂O₇ （A is La or Nd） may have a good performance in the commercial application, but relatively higher cost than that of polycrystalline piezoelectric materials limits its application. Relative to the single crystal, another advantage of polycrystalline piezoelectric ceramic is that they can be changed the synthesis components to control the properties.As a important member of lead-free ferroelectric materials, bismuth layer-structured ferroelectrics （BLSFs）, CaBi₂Nb₂O₉ ceramics gained many attention, because of its relatively high Curie-temperature （T_c）, low dielectric dissipation, and good hysteresis behavior. For those excellent properties, it can be applied in many fields, such as non-volatile random access memories （NvRAMs）, high-temperature piezoelectric devices and optical switches. Doped with moderate tungsten could lift its resistivity to reduce charge leakage effectively. Most of the work is mainly based on the electrical properties of measuring macroscopic properties to study the doping effect, but the changes of microscopic structure or theoretical research is less. In this paper, a series of non-destructive means of spectrum, such as X-ray diffraction（XRD）, Fourier infrared reflection（FIR）, Raman scattering and spectroscopic ellipsometry, are used to analysis the effect of tungsten doped in CaBi₂Nb₂O₉ ceramics of microstructure evolution, phonon, electronic structure and the change of the electron transition. Then, comparing the result of spectrum with macro experiments to find microscopic explanation of tungsten doped effects. At last, the band structure and the density of states CaBi₂Nb₂O₉ are preformed with the first principles calculation to find the theoretical explanation of the microscopic changes.（1） The effects of W subsitution on morphology and microstructure of CaBi₂Nb₂O₉ have been investigated by atomic force microscopy （AFM）, scanning electron microscopy （SEM）, XRD, FIR, Raman spectra.The figures of AFM and SEM show that the surface for samples doped by W are smooth and do not contain cracks with the root-mean square surface roughness of 6nm. It is verified that CaBi₂Nb₂O₉ doped with moderate tungsten could keep single orthorhombic phase. And with the increase the components of tungsten, the structure distortion degree in CaBi₂Nb_2-xW_xO₉ is reduced. The spectra of IR. and Ramn do not appear abnormal peaks in all samples.（2） The lattice vibration in W doped CaBi₂Nb₂O₉ have been investigated by temperature-dependent Fourier infrared reflection, Raman scattering spectra. The relationship between transformation of phonon and the Curie temperature or structure distortion.The changes of lattice vibration with temperature are studied in detail through temperature-dependent Fourier infrared reflection and Raman scattering spectra. Two typical phonon modes, v₅ and v₆, are discussed in detail through Klemens model.The analysis result indicate that the different behaviors of v₅ and v₆ phonon modes during the heating are dominated by thermal expansion of the lattice and unusually positive anharmonic phonon coupling, respectively. Based on the value of the relative peak intensity ratio, it can be inferred that the Curie temperature （Tc） of CaBi₂Nb_2-xW_xO₉ decreases with increasing W composition, which is consistent with the results from temperature dependence dielectric permittivity.（3）The changes of optical energy gap and complex dielectric function, optical band gap are detected by various incident angle spectroscopic ellipsometry spectra. The difference behaviors of transition energy have been explain by the energy band diagram derive from first-principles calculation.The dielectric functions of CaBi₂Nb_2-xW_xO₉ are uniquely extracted in the photonenergy range of 1.8eV-5.5eV with a three-layered model （air/surface roughness layer/ceramic sample）. Through the analysis, the increases of optical band gap with the increase the components of tungsten is introduce of the W 5d electron state. At last, the first-principles calculation result inferred that the changes of structural distortion with tungsten doped mainly caused by the reduce of asymmetry in BO₆, which composed by six chemical bond hybridization by B-site atom and O atom.