Structural properties of the Nb-doped bismuth oxide materials, Bi_1-xNb_xO_1.5+x

Bismuth oxide (Bi₂O₃) exists in five polymorphs, and possesses excellent oxide ion conductivity when in the cubic fluorite structure type, due to its intrinsic oxide ion vacancies. However, this cubic structure is only stable over a small high-temperature range. Introducing niobium into the bismuth oxide structure stabilises the highly conductive cubic and tetragonal phases to room temperature, allowing for high oxide ion conductivity at lower temperatures. In addition to stabilising the high temperature structure types, doping with niobium also introduces interstitial oxygen atoms into the material in order to maintain a charge balance. Niobium-doped bismuth oxide samples, Bi_1-xNb_xO_1.5+x (x = 0.0625, 0.12), were synthesised by a solid state synthetic method, before undergoing AC impedance spectroscopy experiments to study their electrical properties. Both samples showed excellent oxide ion conductivities, with the cubic sample (x = 0.12) possessing higher conductivity values than the tetragonal sample (x = 0.0625). The tetragonal sample does not exhibit a loss in conductivity on thermal cycling, unlike the cubic sample, where the conductivity decreases due to a phase transformation from the cubic to the tetragonal phase. Variable temperature X-ray powder diffraction elucidated the structural transformations which the tetragonal bismuth niobate undergoes; from being tetragonal at room temperature, to cubic above 680 °C, then returning to the tetragonal phase upon cooling. To locate the interstitial oxygen atom positions in the tetragonal phase, powder _neutron diffraction has been undertaken.