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

The pyrochlore-type solid-solution formation in a Bi1.6Mg0.8-xCuxTa1.6O7.2-Delta system, synthesized for the first time, isobserved at x <= 0.56. High-temperature X-ray diffraction showed that the pyrochlore phase exists in air up to 1080 degrees C, where its thermal decomposition leads to the segregation of (Mg,Cu)Ta2O6. The thermal expansion coefficients of the end member, Bi1.6Mg0.24Cu0.56Ta1.6O7.2-Delta, increase from 3.3 x 10(-6) degrees C-1 at room temperature up to 8.7 x 10(-6) degrees C-1 at 930 degrees C. Rietveld refinement confirmed that the pyrochlore crystal structure is disordered with space group Fd (3) over barm: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 theeffective 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 Bi1.6Mg0.8-xCuxTa1.6O7.2-Delta ceramics may achieve up to similar to 10(5), with the dielectric loss tangent varying in the range from 0.2 up to12. Multiple dielectric relaxations are found at room temperature and above for all samples.