Effect of Co2+ substitutions on microstructure and dielectric properties of Bi1.5MgNb1.5O7 cubic pyrochlore

The attractive bismuth-based cubic pyrochlores possess large dielectric constant, high dielectric tunability, and extremely low dielectric loss. The Bi_1.5Mg_1-xCo_xNb_1.5O₇ (Co-BMN, x = 0.0–0.5) cubic pyrochlore ceramics were prepared by solid-state reaction. All the Co-BMN ceramics maintained the cubic pyrochlore phase with a preferential (111) orientation. The forbidden (442) diffraction plane was found in Co-BMN pyrochlores. The crystallization of the Co-BMN ceramics was promoted by Co²⁺ doping modification. The Co-BMN at x = 0.3 achieved the maximum of cell volume, force constant of A-O stretching, and binding energy of Co²⁺. The dielectric constant of Co-BMN significantly increased due to the larger polarizability of Co²⁺. The increasing dielectric constant of Co-BMN pyrochlores (x ≤ 0.3) with increasing Co²⁺ doping was attributed to the displacive disorder of A-site and O’ ions. Co-BMN with moderate Co²⁺ doping (x ≤ 0.4) presented a low dielectric loss (< 0.0007). The temperature coefficient of dielectric constant of Co-BMN pyrochlores (x ≤ 0.4) increased from -382 ppm/^oC to -84 ppm/^oC after Co²⁺ doping. However, high concentration of Co²⁺ resulted in deterioration of dielectric properties of Co-BMN.