P, T and O layers

The index a is usually the number 2 or 3, indicating hep or ccp packing, respectively, and b is a number such that the product a-b gives the total number of close-packed layers in the unit cell. The total number of close-packed layers includes P, T, and O layers. The index b can be a fraction, but the product a-b must be a whole number. 

Here we have briefly considered the structures for various combinations of P, T, and O layers. All patterns for ccp structures are shown in Figure 3.12. The face-centered cube is the same as the cubic close-packed structure. The two names stress particular features. The unit cell of a ccp structure is fee. The packing direction, the direction of stacking the close-packed layers, is along the body diagonals of the cube. The packing direction is vertical in Figures 3.6, 3.7, and 3.8. 

There are two modifications of Agl at ordinary temperatures, (3-AgI has the wurtzite (2 2PT) structure and y-Agl has the zinc blende (3 2PT) structure. For both of these structures Ag+ and I ions have CN 4 (tetrahedral). Above 145.8°C, a-Agl is formed with a bcc (3 2PTOT) structure for I ions. For a bcc structure all P, T, and O layers are filled by I ions for a-Agl. There are secondary interstitial sites for a bcc structure—four distorted tetrahedral sites (T ) in each face of the cube and distorted octahedral sites (O ) in the centers of the edges (12) and in the centers of the faces (6) of the cube. The T sites are shown as squares in each face of the bcc cube in Figure 7.28. The bcc cell... 

In many cases involving partial filling of layers the overall pattern is not clear from the unit cell. Distortions can result from vacancies. Examination of spacings between layers is helpful in identifying P, T, and O layers. In the cases of CdCL, Cdl2, CuBr2, and CrCl3... 

Books using the close-packing description missed the important overall general PTOT pattern. The P, T, and O layers are obvious in hep structures when we look for them, because the layers are packed along the c axis. The layers are not obvious in ccp structures because the common models and drawings show one axis horizontal and one vertical. Layers are packed along the body diagonal of the cell. 

Figure B.8. BiF3 models built from stacking octahedra and tetrahedra and a figure showing the relationships of octahedra, tetrahedra and P, T, and O layers.

Figure B.ll. A side view showing the spacings of P, T, and O layers in a close-...

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