Structure dependence of dielectric properties in Ca-doped bismuth magnesium niobate pyrochlores

Abstract

The cubic pyrochlore Bi1.5MgNb1.5O7 (BMN) materials have large permittivity, good temperature stability, and extremely low dielectric loss. The structure and dielectric properties of Bi1.5Mg1−xCaxNb1.5O7 (BMCN, x = 0.0–0.5) are discussed systematically. All the samples except x = 0.5 were found that the dominant crystalline phase was cubic pyrochlore structure. The BMN (x = 0.0) cubic pyrochlore phase contained little impurity Mg4Nb2O9. Although Ca-doping inhibited the formation of the impurity phase Mg4Nb2O9, excessive Ca-doping (x ≥ 0.4) led to another second phase CaBi2Nb2O9 in BMCN structure. The substitution of Ca2+ brought about the reduction of structure disorder which were caused by abnormal cell shrinkage. It evidenced that Ca2+ ions are prone to substitute Mg2+ ions on A-site, then take on the B-site when the more Ca2+ ions doping in the pyrochlores. The permittivity of doped BMCN (x = 0.1–0.5) was smaller than that of BMN, and decreased with the increase of Ca2+ doping. The dielectric loss of doped BMCN increased due to the lattice defects caused by larger Ca2+ substitution for Mg2+, and the existence of the second phase with x = 0.4–0.5. The temperature dependence of the dielectric properties become unstable by Ca-doping. The Ca2+ substitution led to a closer association of BO6 octahedra, which had adverse impact on temperature stability of BMCN (x = 0.1–0.3). The coexistence of the second phase with positive temperature coefficient improved the dielectric temperature stability of the BMCN (x = 0.4–0.5). The dielectric loss was interpreted on the basis of the interfacial polarization at the grain-boundary region, which would become sensitive to temperature/frequency.