Phase-pure bismuth tantalate fluorites were successfully prepared via conventional solid-state method at 900 °C in 24-48 h. The subsolidus solution was proposed with the general formula of Bi3+x Ta1−x O7−x (0 ≤ x ≤ 0.184), wherein the formation mechanism involved a one-toone replacement of Ta5+ cation by Bi3+ cation within ~4.6 mol% difference. These samples crystallised in a cubic symmetry, space group Fm-3 m with lattice constants, a = b = c in the range 5.4477(± 0.0037)-5.4580(± 0.0039) Å. A slight increment in the unit cell was discernible with increasing Bi2O3 content, and this may attribute to the incorporation of relatively larger Bi3+ cation in the host structure. The linear correlation between lattice parameter and composition variable showed that the Vegard's law was obeyed. Both TGA and DTA analyses showed Bi3+x Ta1−x O7−x samples to be thermally stable as neither phase transition nor weight loss was observed within ~28-1000 °C. The AC impedance study of Bi3TaO7 samples was performed over the frequency range 5-13 MHz. At intermediate temperatures, ~350-850 °C, Bi3+x Ta1−x O7−x solid solution was a modest oxide ion conductor with conductivity, ~10−6-10−3 S cm−1; the activation energy was in the range 0.98-1.08 eV.
Influence of Nb2O5 substitution on the structural and electrical properties of Bi3TaO7 ceramics ABSTRCT Herein we report the doping mechanism and impedance study of the Nb-substituted Bi3Ta1-xNbxO7 (0 ≤ x ≤ 0.5) prepared via conventional solid-state method at 900 °C over 24 h. The substitutional solid solution crystallised in a cubic fluorite structure, space group Fm-3m and with lattice parameter, a = b = c, in the range 5.4477 (±0.0037)-5.4654 (±0.0011) Å. An insignificant unit cell expansion was observed with increasing Nb2O5 content and the linear correlation between lattice parameter and composition variable showed that the Vegard's Law was obeyed. Both TGA and DTA analyses confirmed that the Bi3Ta1-xNbxO7 solid solution was thermally stable as neither phase transition nor weight loss was observed within the studied temperature range, ∼28 °C-1000 °C. The electrical conductivities of these samples were found to increase with increasing Nb concentration; the Bi3Ta0.5Nb0.5O7 exhibited the highest conductivity, ∼1.2 × 10−2 S cm−1 at 700 °C with a low activation energy of 1.03 eV.
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