Spectroscopic study of phase transitions in ferroelectric Bi_0.5Na_0.5Ti_1−xMn_xO_3−δ films with enhanced ferroelectricity and energy storage ability

Lead-free ferroelectric Bi_0.5Na_0.5TiO₃ (BNT) has attracted considerable attention taking into account environment issues and applications in micro electro mechanical systems. The effects of manganese (Mn) substitution on microstructure, lattice dynamics, and optical properties of BNT films have been investigated by X-ray diffraction, Raman scattering and ellipsometric spectra. The phase transitions as a function of temperature have been systemically explored by temperature-dependent Raman and ellipsometric spectra. The anomalous temperature-dependent behavior of Raman-active modes suggests that there is an intermediate phase between ferroelectric rhombohedral and paraelectric tetragonal phase, which is confirmed by the temperature-dependent optical band gap (E_g) and extinction coefficient (κ) extracted from ellipsometric spectra analysis. And then, a phase diagram as a function of Mn composition has been proposed. Finally, we demonstrate that Mn-dopant is an effective approach to enhance ferroelectric properties, improve energy storage ability, and increase permittivity as well as suppress leakage current. Electrical dielectric responses indicate that there is a critical frequency and polarization relaxation behavior in the films. The present results will be helpful for the application of BNT-based lead-free multifunctional devices.