Decrease of the required dopant concentration for delta -Bi sub(2)O sub(3) crystal stabilization through thermal quenching during single-step flame spray pyrolysis

delta -Bi sub(2)O sub(3) is one of the best oxygen ion conductors known. However, due to its limited thermal stability and complicated synthesis techniques, its applications are limited. Here, the synthesis of stable nano-sized delta -Bi sub(2)O sub(3) using versatile and rapid flame spray pyrolysis (FSP) combined with in situ Ti and/or Mn doping for an enhanced thermal stability is reported for the first time. Exceptionally low Bi replacing cation concentrations (8 at% Ti) were sufficient to obtain phase-pure delta -Bi sub(2)O sub(3) which was attributed to the extraordinarily high temperature gradient during FSP. The required cation amount for delta -phase stabilization was even further reduced by introducing mixtures of Mn and Ti (2.5 at% Mn + 2.5 at% Ti). Rietveld analysis revealed that the delta -Bi sub(2)O sub(3) structure is best represented by the Fm3&cmb.macr; m space group containing two closely neighbored 8c and 32f Wyckoff positions. Depending on the amount of Mn/Ti cations, about 25% of the possible oxygen positions remain vacant, suggesting high bulk oxygen mobility. The enhanced oxygen mobility was confirmed by temperature programmed reduction (H sub(2)-TPR) with bulk reduction for delta -Bi sub(2)O sub(3) in contrast to exclusive surface reduction for beta -Bi sub(2)O sub(3).