Site preference of La in Bi_3.75La_0.25Ti ₃O₁₂ using neutron powder diffraction and Raman scattering

Both structural refinement using neutron powder diffraction data and Raman scattering were carried out to determine the site preference of La atoms and the cation distribution in Bi_3.75La_0.25Ti₃O ₁₂ compound. Of three possible cation-disorder models, the best structural refinement result was obtained from a model that La atoms substitute only for Bi atoms outside of the TiO₆ octahedra in the Bi ₂Ti₃O₁₀ unit. The model proposed by the structural refinement was corroborated by the Raman spectroscopic study. The final weighted R-factor, R _wp, and the goodness-of-fit indicator, S (= R _wp/R _e), based on the neutron diffraction and the Raman scattering were 4.12% and 1.43, respectively. The occupancy of La atoms for two Bi sites in the perovskite-like unit was 0.082 and 0.074, respectively. The refined model described a structure in monoclinic space group B1a1 with Z = 4, a = 5.4387(1) Å, b = 5.4129(1) Å, c = 32.8441(1) Å and β = 90.03(1)°.