structures are derived from the ReO3 type.
It consists of ReO6 octahedra which are arranged
in a primitive cubic lattice. The octahedra are linked by corner-sharing
along the axes. In an early HRTEM image of WO3,
which crystallizes in a slightly distorted ReO3 structure,
the square channels are imaged with bright contrast (W. Sahle, Chem.
Commun. University Stockholm, 1983).
the introduction of edge-sharing between octahedra along two
orthogonal directions, the so-called block structures arise.
A block is defined by the number of corner-sharing octahedra
between the shear-planes.The structure of ZnNb14O35F2 (see
image) consists of arrays of 3x5 corner-sharing octahedra ([3x5]
While at the sites of the edge-sharing the distances between
the metal atoms is rather small, leading to high scattering potential
there and therefore to dark contrast in the HRTEM image, the
square channels appear as bright dots. Thus, a [3x5] block appears
in the HRTEM image as an array of 2x4 bright dots.