Hexagonal tungsten bronze synthesized from potassium peroxo-polytungstate and its electrical properties

Peroxo-polytungstic acid, formed by the direct reaction of metallic tungsten with hydrogen peroxide, precipitated an amorphous salt with potassium bromide. Decomposing the precipitate at 400-800°C in air yielded white hexagonal potassium tungstate, K_xWO₃+ x 2 (x=0.3), with lattice parameters of a=7.31 and c=7.65 Å. On the other hand, thermal decomposition in a reducing atmosphere (H₂/H₂O) at 500°C gave dark blue hexagonal tungsten bronze (HTB), K_xWO₃, with a=7.38 and c=7.52 Å, which were almost identical to those reported for HTB. It was found that the redox reaction between tungstate and HTB was completely reversible. A porous pellet of the present HTB showed metallic conductivity (≈10 S/cm, at 25°C) under dry conditions. In a wet atmosphere with only slight humidity, however, its conductivity was dramatically reduced. The conductivity was recovered by treating it again in a dry condition.