Dielectric properties and atomic-scale microstructural characterizations of cubic-pyrochlored ceramics in the system of Bi2O3-MgO-Nb2O5

Abstract

Single-phase pyrochlore-type ceramic samples of Bi1.5MgNb1.5O7 (BMN) in the system of Bi2O3-MgO-Nb2O5 were synthesized via solid-state reactions, and their microstructures, dielectric properties, and leakage current characteristics were also investigated. X-ray diffraction patterns demonstrate the synthesized BMN samples have cubic phase pyrochlore-type structure, which is also confirmed by selected area electron diffraction (SAED) patterns. The highest density of the BMN samples sintered at different temperatures was 6.511 g/cm3, which was about 98.0% of the theoretical one. More than six Raman vibrational peaks observed in the BMN samples also confirmed the cubic phase pyrochlore-type structure of the synthesized BMN samples. TEM images reveal that the BMN samples have clean grain boundaries and no voids are observed at triple-grain boundary. Both SAED patterns and HRTEM images confirm the single crystalline nature of the BMN ceramic grains, and the superlattice structures (or modulated pyrochlore structures) along the [001] and 〈112〉 directions in the BMN samples. Such long-range ordered pyrochlore structures (superlattice structures) could enhance the polarizations of the electric dipoles formed along the superstructure direction, resulting in high dielectric constants of the BMN samples. The dielectric constants of the BMN samples were measured to be 180–195, which exhibit almost frequency independent characteristic in the frequency range of 102–106 Hz. The dielectric losses of the BMN samples are much stable and smaller than 0.002 below 105 Hz, which is ascribed to the existence of clean grain boundaries since they are benefit to reducing the extrinsic dielectric loss. The leakage current values of the BMN ceramics are in the range of 0.13–0.75 μA/cm2 (measured @ electric field E = 13.3 MV/m). A flat frequency independence of the relative dielectric constant, low dielectric loss, and small leakage current characters, make the BMN dielectrics attractive in the fields of high volume efficient multilayered ceramic capacitors with the capacitance weakly dependent on the frequency.