The Superstructure of Lead Tetragonal Tungsten Bronze

Electron-optical studies of the superstructure of lead tetragonal tungsten bronze (lead-TTB) are presented. Samples were synthesized for a range of lead compositions and synthesis conditions. For lead-TTB synthesized for very short reaction times, compositional analysis combined with electron diffraction revealed these specimens to also contain significant levels of an intergrowth tungsten bronze (ITB) phase. It was proposed that ITB was formed during the reaction as an intermediate between the tungsten oxide reagent and lead-TTB. Electron-diffraction investigations of lead-TTB over a range of specimen compositions determined that the large majority of crystallites examined exhibited a well-ordered 2√2a_TTB×√2b_TTB×2c_TTB supercell periodicity, although evidence of a larger a-axis repeat was also observed. No evidence for a variation in the supercell with lead composition was observed. A model for the supercell was generated from consideration of stacking sequences of correlated ordered arrays of lead and tilted octahedra and it was demonstrated that larger superstructures could be generated using alternative stacking sequences.