Nb-doping effect on microstructure, thermal and dielectric properties of bismuth nickel tantalate pyrochlore

Abstract

Mixed nickel pyrochlore of the Bi2NiTaNbO9 composition (sp. gr. Fd-3m:2, a = 10.53242(3) Å, Z = 8) in which half of the tantalum(V) ions are replaced with Nb(V) was synthesized by solid-phase reaction for the first time. X-ray diffraction (XRD) data indicate the formation of a disordered pyrochlore structure with nickel, tantalum and niobium ions distributed in the same system of crystallographic positions. Doping with Nb(V) ions affects the microstructure of the sample. A dense, low-porous ceramic with fuzzy grain boundaries is formed. High-temperature XRD study shows that Bi2NiTaNbO9 is thermally stable up to 1050 °C. The thermal expansion coefficient increases uniformly and weakly from 3.3 to 8.6 × 10−6 °C−1 in the interval 30–960 °C, and its average value is 6.0 × 10−6 °C−1. Bi2NiTaNbO9 pyrochlore can be classified as a weakly expanding compound with isotropic thermal expansion. Doping with niobium had no significant effect on its thermal behavior and stability. By impedance spectroscopy the electrical properties of the samples are investigated. It was found that Bi2NiTaNbO9 exhibits dielectric properties. At room temperature and 1 MHz the sample exhibits dielectric permittivity with the value equals to 40–41 (dielectric loss tangent is 0.01). The sample capacitance weakly depends on temperature and frequency. The conduction activation energy is 1.34 eV. Doping with Nb(V) ions improves the sample dielectric characteristics.