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

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, ex-cessive 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. (C) 2022 Published by Elsevier B.V.

Automated summary

The structure and dielectric properties of BMN and BMCN materials were investigated. It was found that Ca doping influenced the crystal structure and led to the formation of a second phase in BMCN.