Phase stability and microstructure of pyrochlores in the system Bi2O3‑MgO-Ta2O5

Excellent dielectric properties, possibility of regulation of dielectric properties by electric field, low sintering temperature and chemical compatibility with fusible copper conductors make oxide bismuth-containing pyrochlores promising as dielectrics for multilayer ceramic capacitors, tunable microwave dielectric components, and devices for microwave applications. In this work, the concentration stability region of Mg-doped bismuth tantalate pyrochlore was experimentally determined. Based on the study, the conditions for the formation of mixed pyrochlores doped with divalent ions exhibiting a preference for octahedral positions are given. The concentration stability region of magnesium-containing pyrochlores based on bismuth tantalate (Bi2+xMg1+yTa2−yO9−δ, −0.11 ≤ x ≤ 0.325, −0.01 ≤ y ≤0.20) was experimentally determined. On the basis of experimental data, the conditions for the formation of mixed pyrochlores containing ions of divalent elements (Mg(II) type) in Ta(V) octahedral positions were defined. It was shown that phase-pure pyrochlores are formed in the case of a bismuth sublattice vacant, by 37 – 22.5 at.%, and the degree of substitution of cations of the Ta(V) octahedral sublattice by magnesium ions is limited to the interval 33 – 40 %. As SEM studies have shown, an increase in the bismuth content and the Mg / Ta molar ratio in the samples leads to the enlargement of grains and a decrease the pores of the ceramics.