Electronic structures and abnormal phonon behaviors of cobalt-modified Na_0.5Bi_0.5TiO₃-6%BaTiO₃ single crystals

Optical properties, electronic structures, and structural variations of x wt% cobalt (Co) doped Na_0.5Bi_0.5TiO₃-6%BaTiO₃ (x=0%, 0.5%, 0.8%) single crystals have been studied by temperature-dependent optical ellipsometry and Raman spectra from 250 to 650 K. Based on the temperature evolution of electronic transitions (Ecp1 and Ecp2) and the phonon modes involving Ti-O vibrations, two critical temperature points exhibit an increasing trend with Co dopants, which are related to structural variations for ferroelectric to anti-ferroelectric, and anti-ferroelectric to paraelectric transition, respectively. Additionally, distinguishing abnormal phonon behaviors can be observed from Raman spectra for the crystal of x=0.5% and 0.8%, which show reverse frequency shift of the modes involving Ti-O vibration. It can be ascribed to different relative concentration of Co²⁺ and Co³⁺ in the crystals, which has been confirmed by X-ray Photoelectron Spectroscopy data.