Cu, Mg Codoped Bismuth Tantalate Pyrochlores: Crystal Structure, XPS Spectra, Thermal Expansion, and Electrical Properties

The pyrochlore-type solid-solution formation in a Bi_1.6Mg_0.8–xCu_xTa_1.6O_7.2−Δ system, synthesized for the first time, is observed at x ≤ 0.56. High-temperature X-ray diffraction showed that the pyrochlore phase exists in air up to 1080 °C, where its thermal decomposition leads to the segregation of (Mg,Cu)­Ta₂O₆. The thermal expansion coefficients of the end member, Bi_1.6Mg_0.24Cu_0.56Ta_1.6O_7.2−Δ, increase from 3.3 × 10^–6 °C^–1 at room temperature up to 8.7 × 10^–6 °C^–1 at 930 °C. Rietveld refinement confirmed that the pyrochlore crystal structure is disordered with space group Fd3̅m:2 (Z = 8, no. 227). Doping with copper results in a modest expansion of the cubic unit cell, promotes sintering of the ceramic materials, and induces their red-brown color. X-ray photoelectron spectroscopy demonstrated that the states of Bi­(III) and Mg­(II) are not affected by doping, and the effective charge of tantalum cations is lower than +5, while the Cu­(II) states coexist with Cu­(I). The electron spin resonance spectra display a single line with g = 2.2, ascribed to the dipole-broadened Cu²⁺ signal. The dielectric permittivity of Bi_1.6Mg_0.8–xCu_xTa_1.6O_7.2−Δ ceramics may achieve up to ∼10⁵, with the dielectric loss tangent varying in the range from 0.2 up to 12. Multiple dielectric relaxations are found at room temperature and above for all samples.