Thermal Expansion, XPS Spectra, and Structural and Electrical Properties of a New Bi₂NiTa₂O₉ Pyrochlore

A phase-pure nickel bismuth tantalate with pyrochlore structure was synthesized by a solid-phase synthesis method for the first time. The crystal structure of pyrochlore (refined formula Bi_1.58Ni_0.60Ta_1.40O₇, sp. gr. Fd-3m, a = 10.5343 Å, Z = 8) was clarified by the Rietveld method on the basis of X-ray powder diffraction (XRD) data. The crystallite size determined by the Scherrer method is ∼46 nm. The sample has an atypical pink-purple color. The electronic state of the atoms was investigated by XPS. According to XPS analysis, bismuth atoms have an effective charge of +3; nickel atoms, +(2 + δ); tantalum ions, +(5 - δ). The thermal expansion coefficient of the cell is calculated from high-temperature X-ray measurements in the range of 30-1200 °C. The thermal expansion coefficient (TEC) increases monotonically from 3.79 × 10^-6 °C^-1 (30 °C) to 8.32 × 10^-6 °C^-1 (990 °C). Above 1080 °C, the TEC decreases due to the thermal dissociation of pyrochlore with new NiTa₂O₆ phase formation. Ni-doped bismuth tantalate refers to dielectrics and exhibits a moderately high dielectric constant, ∼32, and low dielectric losses, ∼2 × 10^-3 at 1 MHz and ∼30 °C. Above 300 °C, the dielectric losses and dielectric permittivity increase in the low-frequency region due to the activation of oxygen anions. It is found that the electrical characteristics of the sample are significantly affected by the ambient air humidity. An equivalent scheme which satisfactorily describes the electrical properties of the sample has been proposed.