Structural distortion, phonon behavior and electronic transition of Aurivillius layered ferroelectric CaBi₂Nb_2-xW_xO₉ ceramics

The structure and optoelectronic properties of Aurivillius ferroelectric CaBi₂Nb_2-xW_xO₉ (CBNW, x = 0, 0.01, 0.03 and 0.05) ceramics have been studied. X-ray diffraction analysis shows that the ceramics exhibit the pure orthorhombic phase. The W dopant effects on phonon modes, Curie-temperature and optical band gap have been investigated by Raman scattering, temperature dependent dielectric permittivity and spectroscopic ellipsometry, respectively. The different behaviors of 595 cm^-1(ν₅) and 818 cm^-1(ν₆) phonon modes during the heating process are dominated by thermal expansion of the lattice and unusually positive anharmonic phonon coupling, respectively. Moreover, it was found that the distortion degree of (Nb, W)O₆ octahedra in ferroelectric phase of orthorhombic structure can be well discovered by the relative peak intensity of I(ν₆)/I[(ν₅) + I(ν₆)] ratio. Based on the value, it can be also inferred that the Curie temperature of CaBi₂Nb_2-xW_xO₉ decreases with increasing W composition, which is consistent with the results from temperature dependence dielectric permittivity. In addition, it is found that the optical band gap extends with increasing W composition by fitting the ellipsometric spectra with a three-layered model (air/surface roughness layer/ceramic). Meanwhile, the dielectric functions of CBNW ceramics have been uniquely extracted in the photon energy range of 1.8-5.5 eV. To explore the theoretical explanation on the experimental observations, the first-principles calculation on band structure and density of states were performed. It can be concluded that the hybridization between B-site atom orbital and O 2p (or Bi 6p) orbital is weakened by substituting W 5d for Nb 4d, which accounts for the variation of structure distortion and band gap.