Low temperature structural variations of Na_0.5Bi_0.5TiO₃-7%BaTiO₃ single crystal: Evidences from optical ellipsometry and Raman scattering

Optical properties and structural variations of Na_0.5Bi_0.5TiO₃-7%BaTiO₃ (NBT-7%BT) single crystal have been studied by temperature-dependent optical ellipsometry and Raman spectroscopy from 4.2 to 300 K. The second derivative of the complex dielectric functions reveals two interband transitions (E cp 1 and E cp 2) located at about 3.49 and 4.25 eV, respectively. Depending on the temperature evolution of electronic transitions, structural variations appear near 60, 150, and 240 K, respectively. These anomalies are also well illustrated from the low-frequency phonon modes involving vibrations of Bi. The low-temperature structural variations of NBT-7%BT crystal can be associated with instability of the crystalline lattice driven by off-centered Bi ions, followed by the variations of polarizability of the unit cells.