Block Structures I

ReO₃ Structure Type

Block 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 (adopted from: W. Sahle, Chem. Commun. University Stockholm, 1983).

Building Principle

By the introduction of edge-sharing between octahedra along two orthogonal directions, the so-called block structures (also Wadsley−Roth phases) 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] blocks). 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.