Effect of Gd3+/Ti4+heterovalent substitution on the crystal structure, morphology, optical properties, and phase transition behavior of bismuth layer-structured SrBi2Nb2O9

The double-layer Aurivillius compounds Sr1-xGdxBi2Nb2-xTixO9 (x = 0, 0.2, 0.4, 0.6, 0.8, and 1), which have the potential to be developed as a ferroelectric material, have been synthesized using the molten salt method. This study aims to investigate the effect of the simultaneous substitution of Gd3+ and Ti4+ ions in SrBi2Nb2O9 on the phase formation, crystal structure, morphology, and ferroelectric properties. The single-phase product with A21am orthorhombic structure was obtained for all samples by X-ray diffraction analysis. The unit cell becomes more orthorhombic with increasing x as the degree of tetragonality increases. SEM micrographs demonstrate the plate-like grain morphology with an anisotropic growth direction, and the grain is smaller in size with increasing x. FTIR spectra indicate that cation substitution was successful, especially Ti4+ ions at the B-site. With increasing x, the ferroelectric transition temperature (Tc) increases, which has been explained in terms of a reduction in the tetragonality. The open hysteresis loops provide evidence of ferroelectric properties which is consistent with the trend of room-temperature dielectric constant. The optical bandgap value is proportional to the trend of the dielectric characteristics of the samples.