Decrease of the required dopant concentration for δ-Bi2O3 stabilization through thermal quenching during single-step flame spray pyrolysis synthesis

Abstract

<p> <em>&delta;</em>-Bi₂O₃ is one of the best oxygen ion conductors known. However, due to its limited thermal stability and complicated synthesis techniques, the applications are limited. Here, the synthesis of stable nano-sized <em>&delta;</em>-Bi₂O₃ using versatile and rapid flame spray pyrolysis (FSP) combined with <em>in-situ</em> 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 <em>&delta;</em>-Bi₂O₃ which was attributed to the extraordinary high temperature gradient during FSP. The required cation amount for <em>&delta;</em>-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 <em>&delta;</em>-Bi₂O₃ structure is best represented by the <em>Fm</em><em>϶m</em> space group containing two closely neighbored <em>8c</em> and <em>32f</em> <em>Wyckoff</em> 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 (H₂-TPR) with bulk reduction for <em>&delta;</em>-Bi₂O₃ in contrast to exclusive surface reduction for <em>&beta;</em>-Bi₂O_3.</p>