This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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The structure [1] shown in Figure 3469a is formed by a three-dimensional (3-D) Cr₇S₁₂ framework raising the strips of edge-sharing octahedra {CrX₆} as a triple twinning {111} of NaCl-type and thus, resulting in a hexagonal symmetry sublattice. This 3-D framework forms two different types of tunnels with hexagonal and trigonal geometry. The columns with composition A₃CrX₃ located in the hexagonal tunnels and the columns with composition A₃X located in the trigonal tunnels. There are one hexagonal tunnel and two trigonal tunnels in an unit cell. Both types of columns present hexagonal lattices with the same lattice parameters.

Figure 3469a. (a) Ideal structure of PbCr₂S₄ projected onto (0001). Pb, Cr and S atoms are drawn as medium black, small black and big gray circles. (b) The triangular tunnels. Black spheres represent Pb atoms and the gray ones located in the center of the face sharing Pb-prisms are S atoms. (c) The hexagonal tunnels. The black spheres represent Pb atoms and the S atoms are located at the corners of the face-sharing octahedra which are centered by Cr atoms. [2]

Figure 3469b shows the electron diffraction pattern along any [hki0] zone axis. The reflection spots originate from the 3-D framework Cr₂₁S₃₆, while the diffuse diffraction lines originate from the two columnar substructures Pb₆Cr₂S₆ and Pb₃S. The lack of correlation between columns of the same type results in planes of diffuse intensity instead of well-defined reflections, even though some degree of correlation between the columns still exist since the diffuse intensity of the planes are not uniform in the figure.

[1] Hyde, B.G., Andersson, S., 1989. Inorganic Crystal Structures. Wiley, New York.
[2] Landa-Cánovas, A.R., Gómez-Herrero, A., and Carlos Otero-Díaz, L. Electron microscopy study of incommensurate modulated structures in misfit ternary chalcogenides, Micron 32 (2001) 481 - 495.