Structures Involving P and T Layers

The PO structures involve P and O layers with both T layers empty. The most common structure of PO type is the NaCl structure described by 3 2PO. The larger Cl- ions are in the ccp P layers and Na+ ions fill all O layers (Figure 3.6). In a ccp PO structure the P and O layers are identical. We could reverse the labels in the figure, letting the light balls represent Cl- and the dark ones Na+. NiAs is an example of an hep PO structure or 2 2PO, but this structure is not common for hep because of interactions of atoms in O layers, all in C positions (see Section 5.2.1). 

In Figure 3.7, we can see that P and T layers occupy only A and B positions. Let us focus on a Pg layer. Just below and above the Pg layer there are Ta layers. These T sites are very close, with no shielding. For hep structures no examples are encountered for PTT or PTOT with both T layers filled without unusual features (pp. 139-144). Partial filling of both T layers avoids repulsion involving adjacent T sites. 

In the structure of lithium oxide, Li20, as shown in Figure 6.11, there is a ccp (3P) framework of oxide ions with Li+ ions filling both T layers (T+ and T ) between P layers. The very close T+ and T layers (above and below each P layer) are clearly shown. There is no problem with all T sites occupied for a ccp arrangement because the T sites are staggered (A, B, and C). Each oxide ion is at the center of a cube formed by eight Li+ ions. This structure is the reference structure for many related tetrahedral structure involving partial occupancy of T and/or P layers. 

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