Lattice Dynamics, Dielectric Constants, and Phase Diagram of Bismuth Layered Ferroelectric Bi₃Ti_1−xW_xNbO_9+δ Ceramics

Lattice dynamics and phase transitions of Aurivillius ferroelectric Bi₃Ti_1−xW_xNbO_9+δ (BTWN100x, 0 ≤ x ≤ 15%) ceramics have been investigated by temperature-dependent Raman spectra (80–800 K) and far-infrared (FIR) reflectance spectra (6–300 K). The frequency, intensity, and line width of phonon modes as well as complex dielectric functions of BTWN100x ceramics have been extracted by fitting Raman and FIR reflectance spectra with the multi-Lorentzian oscillator model. It was found that the dielectric constants at a certain frequency region become lower with increasing W compositions at room temperature due to the compressive stress on W–O bands. Moreover, the temperature-dependent behavior of optical phonon modes in BTWN100x ceramics indicates that there are two intermediate phases during the phase transition from paraelectric I4/mmm to ferroelectric A2₁am phase on cooling. The phase diagram of BTWN100x ceramics as a function of W composition has been improved.