The crystal structure, thermal and electrical properties of nickel-containing pyrochlore of the Bi2NiNb2O9 composition are studied. The disordered crystalline structure of pyrochlore (sp. gr. Fd-3m, a = 10.53784 Å, Z = 4) is refined by the Rietveld method based on X-ray powder diffraction (XRD) data. In contrast to the loose, porous tantalum pyrochlore (grain size of 0.5-2.0 μm), the microstructure of the niobium ceramics is grainless and low-porous. Niobium pyrochlore has a structure of disordered pyrochlore, in which the bismuth atoms are displaced to the 96g position, and the oxygen atoms are distributed over 48f and 8a with incomplete occupancy. Nickel cations are distributed over the bismuth and niobium positions. The tolerance of the octahedral tantalum sublattice to nickel cations is higher and its capacity with respect to Ni(II) is 5.3% higher than that of the niobium sublattice. At the same time, the vacancy of the bismuth sublattice is 22% compared to 18% for niobium pyrochlore. The thermal expansion of the cell is studied by high-temperature X-ray diffraction in the temperature range of 30-1200 °C. The thermal expansion coefficient (TEC) is higher than for tantalum pyrochlore (from 3.8 × 10−6 to 7.0 × 10−6 °C−1 at 30–750 °C), monotonically increases from 5.12 × 10−6 °С−1 (30°С) to 7.73 × 10−6 °С−1 (750°С). The mechanism of thermal destruction of pyrochlores Bi2NiNb2O9 and Bi2NiTa2O9 is similar and consists in the formation of isostructural intermediate compounds NiNb2O6/NiTa2O6 and the proximity of the thermal dissociation temperature of pyrochlores 1110/1080°С, which is associated with volatility and removal of bismuth(III) oxide at high temperature. The dielectric characteristics of niobium pyrochlore are significantly better than for tantalum-containing pyrochlore of similar composition. Bi2NiNb2O9 is characterized by a high activation energy of 1.43 eV; high frequency-independent dielectric permittivity of ∼144 (up to 300°С) and a dielectric loss tangent of ∼0.002 at 1 MHz. Ion transport was not detected. The investigated ceramics can be used in the multilayer ceramic capacitor creation. © 2026 Elsevier Inc.
