In-situ oxidation products of Nb4W13O47: A high-resolution transmission electron microscopy study

Crystallites of Nb4W13O47 were heated by a focussed electron beam in an oxy gen atmosphere (p(O2) = 20 mbar) inside the gas reaction cell installed wit hin the polepiece of a 400 kilovolt transmission electron microscope. The H RTEM investigation of the resulting oxidation products revealed the presenc e of structures which differ significantly from those generated by conventi onal oxidation in air. In a first reaction step, the arrangement of filled pentagonal tunnels in the tetragonal tungsten bronze (TTB) substructure bec ame disordered, and small segregations of ReO3-type structure arose. Furthe r treatment largely destroyed the original TTB-type structure. In the resid ual bronze-type structure, only a few TTB subcells were preserved while unu sual structural arrangements, including hexagonal and heptagonal tunnels, w ere formed. Moreover, microdomains of a new intergrowth structure between t he TTB type and the ReO3 type occur. This structure comprises double pentag ons of octahedra as well as slabs of diamond-linked pentagonal columns. The oxygen excess in the oxidation product is structurally accommodated in lar ge ReO3-type domains of tungsten oxide.