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

The cubic pyrochlore Bi _1.5MgNb _1.5O ₇ (BMN) materials have large permittivity, good temperature stability, and extremely low dielectric loss. The structure and dielectric properties of Bi _1.5Mg _1−xCa _xNb _1.5O ₇ (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 Mg ₄Nb ₂O ₉. Although Ca-doping inhibited the formation of the impurity phase Mg ₄Nb ₂O ₉, excessive Ca-doping (x ≥ 0.4) led to another second phase CaBi ₂Nb ₂O ₉ in BMCN structure. The substitution of Ca ²⁺ brought about the reduction of structure disorder which were caused by abnormal cell shrinkage. It evidenced that Ca ²⁺ ions are prone to substitute Mg ²⁺ ions on A-site, then take on the B-site when the more Ca ²⁺ 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 Ca ²⁺ doping. The dielectric loss of doped BMCN increased due to the lattice defects caused by larger Ca ²⁺ substitution for Mg ²⁺ , 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 Ca ²⁺ substitution led to a closer association of BO ₆ 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.