Structural refinement using both neutron powder diffraction data and Raman
spectroscopy were carried out to determine the site preference of Ce atoms
and structural changes in Bi4−xCexTi3O12 (x = 0, 0.25, 0.5 and 0.75)
compounds. In the Raman spectroscopic study, the peak at 65 cm−1 of
Bi4Ti3O12 (BTO), assigned to the Bi–O bonds in the Bi2O2 layers, did not
shift or shifted to 63 cm−1 after Ce incorporation. However, the peaks of
BTO at 119 and 148 cm−1, assigned to the Bi–O bonds in the perovskite
units, became diffusive and moved to higher frequencies of (121 and
150 cm−1), (123 and 152 cm−1) and (125 and 157 cm−1) when x was 0.25,
0.5 and 0.75, respectively. From the Raman spectroscopic study, the
substitution sites of the Ce atoms were determined to be only the Bi sites in
the perovskite units. The peaks originated from the TiO6 octahedrons
moved to higher frequencies and merged with the neighbouring peaks,
indicating relaxation of distortion of TiO6 octahedrons with increase in x.
Structural refinement results showed that the a-axis lattice parameters
decreased with increase in x, while the b-axis lattice parameters did not
exhibit significant changes and the c-axis lattice parameters decreased.
Decrease in orthorhombicity with increasing x showed the relaxation of
structural distortion of TiO6 octahedrons. Spontaneous polarization was
calculated based on the refinement results, which decreased from 35.4 to
30.4, 27.5 and 16.5μCcm−2 when x is 0, 0.25, 0.5 and 0.75,
respectively.