Intrinsic relationship between electronic structures and phase transition of SrBi_2-xNd_xNb₂O₉ ceramics from ultraviolet ellipsometry at elevated temperatures

The ferroelectric orthorhombic to paraelectric tetragonal phase transition of SrBi_2-xNd_xNb₂O₉ (x = 0, 0.05, 0.1, and 0.2) layer-structured ceramics has been investigated by temperature-dependent spectroscopic ellipsometry. Based on the analysis of dielectric functions from 0 to 500 °C with double Tauc-Lorentz dispersion model, the interband transitions located at ultraviolet region have shown an abrupt variation near the Curie temperature. The changes of dielectric functions are mainly due to the thermal-optical and/or photoelastic effect. Moreover, the characteristic alteration in interband transitions can be ascribed to distortion of NbO₆ octahedron and variation of hybridization between Bi 6s and O 2p states during the structure transformation.