The Study Of Optical Properties And Phase Transition Of Bi₅Ti₃FeO₁₅ Ceramics

In recent years, the Aurivillius family of bismuth layer structured compounds with general formula (Bi2O2)2+(An-1BnO3n-1)2- has attracted much attention due to the remarkable multiferroic, lead/fatigue-free characteristics and high Curie temperature (Tc). These excellent properties make it potential applications as non-volatile ferroelectricrandom access memories (FeRAMs) and high-temperature and high-frequency piezoelectric materials. Compared with the highly competitive multiferroic BiFeO3, the layer structured Bi5Ti3FeO15 (BTF) can substantially reduce leakage current and reach high resistivity at room temperature (RT) because Fe-centered octahedron is inserted into two adjacent Bi-O layers. BTF ceramic presents a coexistence of antiferromagnetic (AFM) and ferroelectric (FE) order parameters:an AFM Neel temperature (TN) of about 80 K and a FE Tc of about 1023 K. The coupling between AFM and FE properties makes it possible for manipulating electrical property through magnetic fields and vice versa. Nevertheless, the physical mechanism of AFM to PM phase transition still remains ambiguous, which is a challenging problem for further exploitation of multiferroics based device applications. Microscopic nature of the phase transition is required for the optimization of multiferroic properties. In this paper, variable-temperature spectroscopic ellipsometry, Raman scattering, Fourier transform infrared (FTIR) reflectance spectral experiments were carried out to show the process of phase transition, analyze the change of microstructure, and study the mechanism of phase transition.1. Thermal evolution and an intermediate phase between ferroelectric orthorhombic and paraelectric tetragonal phase of multiferroic BTF ceramic have been investigated by temperature dependent spectroscopic ellipsometry and Raman scattering. Dielectric functions and interband transitions extracted from the standard critical-point model show two dramatic anomalies in the temperature range of 200-873 K. It was found that the anomalous temperature dependence of electronic transition energies and Raman mode frequencies around 800 K can be ascribed to intermediate phase transformation. Moreover, the disappearance of electronic transition around 3 eV at 590 K is associated with the conductive property.2. Optical phonons of multiferroic BTF ceramic have been investigated by low temperature Raman scattering and infrared reflectance spectra. Anomalies at about 85K can be observed from the temperature dependence of the Raman and infrared modes, which arise from spin-phonon interaction during antiferromagnetic to paramagnetic phase transition. It was found that the change of exchange interaction in magnetic phase transition can be induced by Fe-O-Fe octahedral tilting driven from the A-site atoms. Moreover, ferroelectricity-related displacement of Bismuth atoms suggests the coupling of magnetic and ferroelectric orders.