Examples of substitutional superstructures

A first group of superstructures, described in several paragraphs of this chapter and of Chapter 7, must be mentioned these include the types tP2-AuCu(I), cP4-AuCu3 and tP4-Ti3Cu which can be considered face-centred cubic-based substitutional ordered superstructures. 

The tI10-MoNi4 type is another superstructure based on face-centred cubic pseudo-cells. In the projection shown in Fig. 3.36, inside the true cell, the pseudo-cubic subcell (aps = 362 pm, cps = 356.4 pm) has been evidenced by dotted lines. Close-packed layers can be identified in this structure they are stacked in a 15 close-packed repeat sequence. 

Like the Au-Cu system, also the alloy system Cu-Pt is characterized by a phase diagram with the face-centred cubic continuous solid solution stable at high temperature and, for different composition ranges, a number of ordered superstructure phases stable at lower temperatures. CuPt(I), for instance, is a complex, slightly 

Another example of a superstructure based on a close-packed structure but related to the hexagonal close-packed one is that corresponding to the hP8-Ni3Sn prototype. Just as the AuCu3 type can be derived by ordering the Cu-type structure, so the Ni3Sn type can be obtained from the hP2-Mg type. Details of this structure and of some stacking variants are described in Chapter 7. 

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