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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In the two dimensional (2-D) case, a periodic structure with one atom at the origin of the unit cell is shown in Figure 3489 (a). A twofold superstructure corresponding to a doubled a₂ axis can be formed by atomic displacements with equal amounts, alternating into the directions ±a₂ as shown in Figure 3489 (b). Therefore, the new structure has a long-range order and translational symmetry, but the volume of the unit cell is twice the original unit cell. This superstructure can be described as a commensurately modulated structure.

Figure 3489. (a) Basic structure indicated by the red lines.
(b) Twofold superstructure with the supercell indicated by red lines.