Cation disorder study of Bb3.25La0.75Ti3O12 by neutron powder diffraction and Raman spectroscopy

Structural refinement and Raman spectroscopy were used to obtain structural information for Bi3.25La0.75Ti3O12, including site preference of La atoms, cation distribution, cation occupancy and lattice parameters. Structural refinement was carried out for three cation disorder models. The best structural refinement result was obtained assuming that, of the three possible substitutions, La atoms substitute Bi atoms only in the perovskite Bi2Ti3O10 unit. The final weighted R-factor, R-wp, and the goodness-of-fit indicator, S (=R-wp/ R-e), were 4.81% and 1.56%, respectively. The results of Raman spectroscopy provided evidence for the cation disorder model. The band originating from the Bi atoms in the Bi2O2 layer did not show significant changes while the vibrational modes assigned to the Bi atoms in the perovskite units moved to high frequencies due to the substitution of La atoms. The final lattice parameters obtained from the refinement were a = 5.4238(1) Angstrom, b = 5.4169(2) Angstrom and c = 32.9002(2) Angstrom. The beta angle was 90.09degrees.