The pyrochlore-type solid-solution
formation in a Bi1.6Mg0.8–x
Cu
x
Ta1.6O7.2−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, Bi1.6Mg0.24Cu0.56Ta1.6O7.2−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 Bi1.6Mg0.8–x
Cu
x
Ta1.6O7.2−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.