La3+ substitution induced structural transformation in CaBi4Ti4O15 Aurivillius phases: Synthesis, morphology, dielectric and optical properties

In this work, La-substituted CaBi4Ti4O15 Aurivillius phase, CaBi4-xLaxTi4O15 (x = 0, 0.5, 1, 1.5, and 2), were synthesized by a molten salt method. The effects of La3+ substitution on structure, morphology, dielectric and optical properties were systematically investigated. Refinement of XRD data confirm that a ferroelectric orthorhombic structure with space group A2(1)am was adopted for x = 0, 0.5, 1, and 1.5. In terms of x = 2, the structure transformed to an intermediate paraelectric orthorhombic structure of the Amam space group. The anisotropic plate-like grains were observed across all the samples, which decreased in size with an increase in La3+ content. With La3+ substitution, the ferroelectric transition temperature (T-c) significantly decreased since the structural distortion as the effect of 6s(2) lone pair electron of Bi3+ was reduced with the substitution of La3+. Additionally, the reduced structural distortion was responsible for a decrease in the bandgap (E-g) energy. A diffused phase transition of ferroelectric characterized by the broadened T-c peak was induced by La3+ substitution since the increased disorder of A-site cation (Ca/Bi/La). The ferroelectric phase with a lower T-c and the lower E-g energy proposes the sample of x = 1.5 as a potential material for electrocaloric and solar cell applications.