Structural and Dielectric Characterization of SrBi$_{2}$Ta$_{2}$O$_{9}$ Doped with Gadolinium

The results of the study of polycrystalline samples of SrBi$_{2}$Ta$_{2}$O$_{9}$ (SBT) doped with Gd, Sr-vacancies and a Bi-excess are presented. XRD structural characterization using Rietveld refinement revealed that the Gd-doped SBT with Sr-vacancies and Bi-excess (Sr$_{0.70}$?$_{0.15}$Gd$_{0.15}$Bi$_{2}$Ta$_{2}$O$_{9})$ is single phase. Furthermore, cationic disorder on the Bi$^{3+}$ and Sr$^{2+}$ crystallographic sites is promoted by the addition of the Gd$^{3+}$ ions. Replacement of Gd ions for Sr and vacancies in the SBT structure produces a shift toward lower values of the Curie Temperature (Tc) from 400 to 200 $^{o}$C in contrast with previous reports in the literature where a notable increase in Tc and the dielectric permittivity are found when the Sr- site is replaced with vacancies. Bi and Gd substitution and the cation vacancies at the Sr site in the Sr$_{0.70}$?$_{0.15}$Gd$_{0.15}$Bi$_{2}$Ta$_{2}$O$_{9}$ compound, enhance the structural distortion in the TaO$_{6}$ octahedra and leads to a larger ferroelectric spontaneous polarization. Acknowledgments: This work was partially supported by DGAPA-UNAM Project N$^{o}$ IN100903 and CONACYT No. 47714-F and 40604-F. The authors thank Pedro Casillas, M. Sainz, J. Peralta for their technical assistance.