Structural and dielectric properties of Sr3(MgTa2)O9 and Sr3(ZnTa2)O9

Herein, we report the crystal structures and morphological properties of Sr-3(MgTa2)O-9 (SMT) and Sr-3(ZnTa2)O-9 (SZT) synthesized by solid state ceramic method along with the results of alternating current impedance spectroscopic (ACFS) study in a frequency range from 50 Hz to 1 MHz at selective temperatures between 393 and 573 K. The crystal structures of SMT and SZT have been determined by Rietveld refinement of powder X-ray diffraction pattern using an initial structural model developed On the basis of literature survey. The results indicate that both the samples possess hexagonal structure of trigonal P (3) over bar m1 space group. The lattice parameters of SMT are a=b=5.65162 angstrom, c=6.94440 A, alpha-beta-90 degrees and gamma=120 degrees and those of SZT are a=b=5.65832 angstrom, c=6.95911 A and alpha=beta=90 degrees and gamma=120 degrees. SMT and SZT are isostructural and they exhibit 2:1 B site ordering with the staking sequence of {-Ta-Ta-Mg (Zn)-} (Mg for SMT and Zn for SZT) layer repeat on (111) plane of the pseudocells. The characteristic vibrational bands due to Ta-O, Mg-O and Zn-O bonds have been observed in the FUR spectra of the samples. The FESEM micrographs of the samples show that the grains size ranges between 0.40 and 3.65 mu m and 0.9 to 4.2 mu m for SMT and SZT, respectively. To account for the polydispersive nature of the dielectric relaxation mechanism along with the effects of dc conductivity and localized space charges the variation of real ((7) and imaginary (epsilon '') parts of dielectric constant with frequency has been analytically interpreted in the framework of modified Cole-Cole model. SMT and SZT having the activation energies of 0.35 eV and 0.33 eV, respectively (obtained from the Arrhenius plot of dc conductivity), are semiconducting in nature. The electrical current conduction in the samples occurs by polaron hopping process. Further, we have shown that chemical property of A site cations has significant role in determining the dielectric properties of A(3)B'B '' O-2(5) type perovskites and these properties do not change appreciably upon replacement of the divalent B' cations. Moreover, owing to their high dielectric constant and low dielectric loss SMT and SZT appear to be potent candidates for technological applications in radio-frequency devices. (C) 2015 Elsevier B.V. All rights reserved.