Effects of Ni2+ substitution on the structure and dielectric properties of Bi1.5MgNb1.5O7 cubic pyrochlores

The dielectric properties of bismuth-based cubic pyrochlores strongly depend on the environment of the A-site ions, e.g. the Ni2+ ions doped into Bi1.5MgNb1.5O7 (BMN) pyrochlores for tailoring dielectric properties. Both BMN and Bi1.5NiNb1.5O7 (BNN) ceramics exhibit a cubic pyrochlore structure with preferential (222) planes. However, {442} reflections are observed in BNN pyrochlores, revealing an off-center displacement of A and O' ions. The dielectric constant of BNN pyrochlores is lower than that of BMN pyrochlores, besides BNN pyrochlores have a larger dielectric loss (0.002) than BMN pyrochlores (0.0007). Ni-doping results in a loose and flexible structure contributing positively to the dielectric tunability, besides creating a large amount of oxygen vacancies. The higher amount of oxygen vacancies increases the dielectric loss of BNN pyrochlores. However, BNN pyrochlores exhibit enhanced temperature stability, with a temperature coefficient of –57 ppm/oC, which is significantly better than that of BMN pyrochlores (–362 ppm/oC).