Practical Electron Microscopy and Database

Superstructures can appear in company with disordering or less-ordering of some atomic arrangements. For instance, two niobium tungsten oxides (Nb₇W₁₀O_47.5 and Nb₄W₁₃O₄₉) consist often of less-ordered arrangements of filled tunnels, causing the diffuse scattering in electron diffraction. Figure 2327a shows the electron diffraction patterns (along [001]) of two crystallites of Nb₇W₁₀O_47.5 presenting circular diffuse scattering in different degrees, which appears around the main bright reflections of the basic TTB substructure. In Nb₄W₁₃O₄₉ as shown in Figure 2327b, the diffuse scattering pattern is cross-shaped due to the presence of long slabs of diamond-linked pentagonal columns. Overall, the origin of the diffuse scatterings is presumably the presence of short-range order in the PC (pentagonal columns) sublattice [1], or an intermediate state of order (or called transition state [2]) between a random arrangement and a long-range ordered structure. In addition, the weak spots indicate TTB superstructures.

Figure 2327a. The electron diffraction patterns (along [001]) of two crystallites of Nb₇W₁₀O_47.5. Adapted from [3]

Figure 2327b. The electron diffraction patterns (along [001]) of a Nb₄W₁₃O₄₉ crystallite. Adapted from [3]

Figure 2327c shows the [100] electron diffraction pattern and intensity distribution in the reciprocal lattice of TiN_0.61.

Figure 2327c. The [100] electron diffraction pattern and intensity distribution in the reciprocal lattice of TiN_0.61. The large circles in (b) show the fundamental reflexions, while the small circles in (b) show the superstructure reflexions. Adapted from [4]

[1] Iijima, S. & Cowley, J. M. (1977). J. Phys. Colloq. 38, 135-144.
[2] Amelinckx, S. (1992). Materials Science and Technology, Vol. 2A, Characterisation of Materials, Part. I, pp. 1-146. Weinheim: Verlag Chemie.
[3] Frank Krumeich, Order and Disorder in Niobium Tungsten Oxides of the Tetragonal Tungsten Bronze Type, Acta Cryst. (1998). B54, 240-249.
[4] S. Nagakura and T. Kusunoki, Journal of Applied Crystallography 10 (1977) 52.