Optical Properties And Phase Transition Of Sr_xBa_1-xNb₂O₆ Ceramics

In recent years, unfilled tungsten bronze lead-free ferroelectric material SrxBa1-xNb2O6(SBN) has attracted extensive research due to its excellent ferroelectric, electric-optics, and pyroelectricity properties. Currently, the research of lead-free ferroelectric materials has been focused on three areas:1) Bi-layer structure based ferroelectrics, such as Bi4Ti3O12, SrBi2Nb2O9(SNBO), etc.2) perovskite type titanate-based ferroelectrics, such as BaTiO3(BT), SrTiO3(ST), and (BaxSr1-x)TiO(BST), etc.3) tungsten bronze type niobate-based ferroelectrics, such as LiNbO3, Sr,Ba1-xNb2O6(SBN), etc. Among them, SBN has a great application prospect in electro-optic detector, pyroelectric detector, and ferroelectric nonvolatile memory because of its high resistivity, Good fatigue resistance characteristic, and high dielectric constant. The special frame of unfilled tungsten bronze ferroelectric make it possible to adjust its phase transition temperature with different Sr/Ba ratio and doping other metallic element. To reduce the phase transition temperature and promote the performance of relevant device have become one of the research objectives that researchers are devoted to. However, most of the existing work have adopted microstructure based XRD diffraction or electrical method to invest the macro characteristics. We lack investigations of its optical properties a lot. We can use optical methods to obtain materials’dielectric functions and fit the results to acquire the electron transition information. It would be of great significance to invest materials’mechanism of phase transition combined with infrared and Raman phonon vibration information. Thus, we investigate the lattice vibration and electronic properties of Cay(SrxBa-x)1-yNb2O6 ceramics with different Sr/Ba ratios and different Ca dopants by temperature-dependent Raman spectroscopy and spectroscopic ellipsometry experiment in this article. The phase transition from ferroelectric to paraelectric can be detected from the main mode variation with temperature. Moreover, the relationship between the energy band gap and phase transition of SBN ceramics is discussed in detail.(1) The phase transition of SBN and Ca doped SBN from ferroelectric to paraelectric has been investigated through temperature dependent Raman scattering in the temperature range of 150-750 K. The frequency of phonon mode locates at about cm" move to low frequency, full width at half maximum (FWHM) increase and vibration intensity die down with increasing the temperature. The temperature coefficient of the frequency and FWHM of cm" Raman mode modifies in the vicinity of Curie temperature after fitting procedure. The Curie temperature of SBN ceramics decreases from 556 K to 359 K with increasing Sr content. The phenomenon can be attributed to the A site substitution that originated in the smaller ionic radius of Sr, as compared to the Ba element. The difference in the Ba-O and Sr-O bond distance and the presence of vacancies can lead to a large distortion of the octahedron. As for CSBN ceramics, however, the Tc changes slightly and almost remains unchanged at about K, which is consistent with the results from the electrical method. This could be due to the reason that Ca+ ionic move into C site which is formally occupied by O atoms and contribute little to displacement along z axis. While the displacement along z axis is proportional to the Curie temperature. The relationship between Sr content and Curie temperature can be interpreted by:Tc(x)= 860-1000x.(2) The complex dielectric functions of SBN and Ca doped SBN ceramics have been investigated with temperature dependent spectroscopic ellipsometry (SE) in the temperature range of 200-600 K. We use Tauc-Lorentz model to fit it and find a typical interband transition at about 5 eV, and it decreases with increasing temperature. After investigation of the relationship between transition energy and temperature, we find two abnormal points between 200-600 K. This temperature region can be interpreted as the diffuse phase transition of SBN. Moreover, good agreement between both kinds of measurements is obtained, corroborating the validity of spectroscopic ellipsometry for measuring the Curie temperature of SBN ceramics. These results indicate that SE could be a possible method to identify the phase transition of relaxor ferroelectric materials.