Hexagonal stacking sequence

The As atoms form close-packed layers in the hexagonal stacking sequence. .. BC. The Ni atoms (on sites A) are placed in the centres of all the octahedral interstices in the As layers. Ni has 6 As neighbours and As is surrounded by 6 Ni (forming a right trigonal prism). 

There are three possible arrangements of atoms in a layer of SiC crystal, and each type has the same layers but a different stacking sequence (29). Designation (30) is by the number of layers in the sequence, followed by H, R, or C to indicate whether the type belongs to the hexagonal, rhombohedral, or cubic class. 

Here, we have arranged the layers on a two-dimensional structure, even though the layers are arranged in three dimensional order. Note that only two crystallographic axes are indicated. We call this the natural stacking sequence because of the nature of the hexagonal close- packed lattice. 

Another tj je of stacking fault is called "poljretructure". A good example is ZnS, which is dimorphic (has two forms). The cubic form of ZnS is called sphalerite, whereas the hexagonal form is CciUed wurtzite (These are their mineral names, aifter the first geologist who discovered them). The stacking sequence for sphalerite is AB or ABBA that for wurtzite is ABC. A polystructure sometimes results when sphalerite is converted to wurtzite ... 

Tc or c cubic closest-packing of spheres Th or h hexagonal closest-packing of spheres Ts stacking sequence AA... of hexagonal layers Qs stacking sequence AA... of square layers... 

Aside from the ordered stacking sequences we have considered so far, a more or less statistical sequence of hexagonal layers can also occur. Since there is some kind of an ordering principle on the one hand, but on the other hand the periodical order is missing in the stacking direction, this is called an order-disorder (OD) structure with stacking faults. In this particular case, it is a one-dimensionally disordered structure, since the order is missing only in one dimension. When cobalt is cooled from 500 °C it exhibits this kind of disorder. 

Usually, the composition of the compound is fulfilled in all of the hexagonal layers. This facilitates a rational classification of the extensive data it is only necessary to draw a sketch of the atomic arrangement in one layer and to specify the stacking sequence... 

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