Study of fluorite phases in the system Bi2O3–Nb2O5–Ta2O5. Synthesis by mechanochemical activation assisted methods

Abstract

Mechanochemical activation followed by annealing at moderate temperatures results in the stabilisation, at room temperature, of different fluorite-type phases, belonging to the Bi₂O₃–Nb₂O₅, Bi₂O₃–Ta₂O₅ and Bi₂O₃–Nb₂O₅–Ta₂O₅ systems. The results obtained from different starting compositions and mechanical activation devices (vibrating and planetary ball mills) were compared with those obtained by classical solid-state synthesis methods. Vibrating ball mill activation yields amorphous precursors, which permits one to obtain fluorites with increasing pentavalent cation content when the annealing temperature is further increased. Planetary ball milling leads to the apparent mechanosynthesis of a fluorite phase at room temperature. The products were studied by X-ray powder diffraction at room temperature and above, thermal analysis techniques and transmission electron microscopy. Moreover, impedance spectroscopy measurements carried out on Bi₃MO₇ (M = Nb, Ta) fluorites, obtained by a mechanochemically assisted method, showed that these materials are good ionic conductors, with conductivities at 600 °C of 5 × 10⁻⁴ S cm⁻¹ or higher, the processing history of the materials having a great influence on their properties.