Block structures
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 3×5 corner-sharing octahedra ([3×5] 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 [3×5] block appears in the HRTEM image as an array of 2×4 bright dots.
Block structures II
A variety of niobium oxides and Nb2O5-rich phases form related block structures. In solid state chemistry, such structures were a major research topic in the 1960s and 1970s. Around 1970, TEMs with a resolution of better than 0.4 nm became available, which was sufficient to resolve the channels in ReO3-type compounds (distance about 0.38 nm). This constitutes the beginning of HRTEM in solid state chemistry. Besides the investigation of ideal structures, that of real structures became possible and increasingly important. Two examples, recorded with a Philips EM400 microscope (point resolution about 0.35 nm), are shown and shortly discussed here.
ZnNb14O35F2: Area with Wadsley Defect
F. Krumeich, R. Gruehn, Z. anorg. allg. Chem. 570 (1989) 133-137 external page DOI
(Mg,Nb)12O29
F. Krumeich, R. Gruehn, Z. anorg. allg. Chem. 564 (1988) 7-16 external page DOI)
Applications
While niobium oxides were a topic of purely academic research for many decades, they have now turned into materials of broad interest because of their reversible, fast, and stable Li uptake properties (external page review).