Cation disorder study of ferroelectric Bi3.5La0.5Ti3O12 by combined refinement of neutron and x-ray powder diffraction data

Combined structural refinement of Bi3.5La0.5Ti3O12 against both neutron and X-ray diffraction data was performed at 298 K based on the Raman spectroscopic study. The up-shift of Raman peaks suggested that the substitution sites of La atoms in Bi3.5La0.5Ti3O12 were only the Bi sites in the perovskite units. Of the two crystal structural models, orthorhombic and monoclinic systems, considered as the crystal structural system of Bi3.5La0.5Ti3O12, the weighted R-factor, Rwp, and goodness-of-fit indicator, S ( = Rwp / Re), of the monoclinic system were lower than those of the orthorhombic one. The final Rwp, and S based on the monoclinic system was 7.04 % (6.34 % and 7.76 % for the neutron data and the X-ray data, respectively) and 1.45, respectively. The lattice parameters obtained from the combined structural refinement were a = 5.4321(1) A , b 5.4161(1) A , and c = 32.8614(3) A . The β angle was 89.95(4) .deg.. Spontaneous polarizations calculated from the refined structural parameters were 27.0 μC/cm2 for the monoclinic system and 1.8 μC/cm2 for the orthorhombic one, respectively