Hexagonal packing, faults

Figure 6. TEM images of STAC-1 viewed down the a axis of a hexagonal unit cell (indicated by [M/]h) or the [110] direction of a cubic unit cell (indicated by [M/]c). The crystal is dominated by ABCABC close packing (indicated on (a)) with one stacking fault (marked by a horizontal line). A Fourier transform optical diffraction pattern with both Miller-Bravais indices to the hexagonal unit cell and Miller indices (in parentheses) to the cubic unit cell is inserted in (b). Simulated images based on a proposed model (right) are also inserted with specimen thickness of 30 nm, and lens focuses of—30 nm (a) and —10 nm (b).

We should briefly consider the changes that happen to the low Miller index planes of gold these have been deeply researched, and are quite complex, so a simple summary must suffice. Gold is the only element the (111) surface of which reconstructs under UHV conditions.50 The new structure is described by a complex stacking-fault-domain model in which there are areas (or domains) of both fee and eph (close-packed hexagonal) structure its Miller index is (23 x V3), and 23 atoms occupy positions that would normally be taken by 22 atoms, and in consequence the new surface is... 

A second type of boundary, in which there is no misorientation between grains, is the antiphase boundary. This occurs when wrong atoms are next to each other on the boundary plane. For example, with hexagonal close-packed (HCP) crystals, the sequence. .. ABABAB... can be reversed at the boundary to ABABA ABABA, where represents the boundary plane. Antiphase boundaries and stacking faults are typically of very low energy, comparable to that of a coherent twin boundary. 

Recently, Rocha and Zanchet have studied the defects in silver nanoprisms in some detail and have shown that the internal structure can be very complex with many twins and stacking faults [107]. These defects are parallel to each other and the flat 111 face of the nanoprism, subdividing it into lamellae which are stacked in a <111> direction, and are also present in the silver seeds. In that paper, it was demonstrated how the planar defects in the <111> direction could give rise to local hexagonally close-packed (hep) regions. These could in turn explain the 2.50 A lattice fringes that are observed in <111> orientated nanofrisms, which have hitherto been attributed to formally forbidden 1/3(422 reflections as mentioned above. 

Thus, the stacking fault patterns noted are a function of the t5q>e of lattice involved, not on the chemical composition of the material. For the most part, these stacking faults are found only in the high symmetry lattices such as hexagonal close-packed and cubic close-packed structures. 

Partial dislocations can form not only by splitting a perfect dislocation, but also by inserting or partly removing a 111 plane. In Fig. 3.58a, the FCC stacking of slip planes is illustrated schematically before the removal of a plane. The sequence of the stacking of the 111 planes is modihed at the region where the fault exists from the FCC to a hexagonal closely-packed [henceforth HCP] structure, as seen in Fig. 3.58b. 

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