Temperature-dependent phonon Raman scattering and spectroscopic ellipsometry of pure and Ca-doped Sr_xBa_1-xNb₂O₆ ferroelectric ceramics across the phase transition region

Optical phonons and the phase transition of relaxor ferroelectric ceramics Sr_xBa_1-xNb₂O₆ (SBN) and Ca_y(Sr_0.5Ba_0.5)_1-yNb₂O₆ (CSBN) with different composition (, ) have been investigated by variable-temperature Raman scattering and spectroscopic ellipsometry. The anomalous temperature dependence of Tauc gap energy (E _t) is used to fit the phonon energy dependence of the permittivity, and the Raman intensity of some interesting optical phonons can be ascribed to the phase transition from a ferroelectric to a paraelectric structure. The Curie temperature of SBN decreases from 556 to 359 K with increasing Sr composition, which can be attributed to the substitution of smaller Sr²⁺ for Ba²⁺. On increasing the Ca composition, however, the phase transition temperature of CSBN remains nearly unchanged at about 350 K. This could be due to the fact that most doped Ca²⁺ ions move into the oxygen ion site and exhibit no obvious effect on the vibrational properties. Therefore, the general disorder which results from Sr²⁺ substituting Ba²⁺ , dominates the phase transition process for SBN-based ferroelectric oxides. Meanwhile, the dielectric functions from 200 to 600 K have been evaluated with the aid of the Tauc-Lorentz model. The electronic transition is located at about 5 eV and decreases with increasing temperature for all the samples. Moreover, the phase transition temperature range derived from the spectroscopic ellipsometry agrees well with that from the Raman scattering. It reveals that the variation of the fundamental energy gap may be associated with the phase transition of SBN ceramics. Both Raman scattering and spectroscopic ellipsometry are proven to be a effective method of exploring the phase transition of ferroelectric oxides.