Structures Involving P and O Layers

We begin consideration of structures of compounds as various combinations of layers. In this chapter we examine compounds that involve only P and O layers. These structures include hundreds of MX type compounds and, with partial filling of layers, compounds other than those with 1 1 atom ratios. Octahedral layers are normally halfway between P layers, and for a ccp arrangement the P and O layers are equivalent and can be interchanged. Ionic MX compounds are commonly encountered for PO structures. The configuration of neighbors of atoms in O sites are normally octahedral, but can be square planar or linear for partial filling of layers. 

The structure of NaCl, table salt, rock salt, or halite (name of the mineral) is the most important and most common one for MX-type salts. Hundreds of salts have this structure, and many examples are listed in Table 5.1. It is characteristic of ionic MX-type compounds except for those with large differences in sizes of cations and anions. The coordination number (CN) is six for both ions for NaCl. Large cations, particularly with small anions, prefer a larger CN, commonly CN = 8 as in the CsCl structure. Small cations, and particularly for compounds with significant covalent character, prefer the CN = 4 as in ZnS. 

MnSe NH4Br NH4CI NH4I NaBr NaCN NaCl 

Mg02 NiS2 CoAsS RhSe2 OsTe2 PtAs2 PtBi2 

The 3 2PO structure of [Co(NH3)6][T1C16] is an NaCl-type arrangement with the octahedral ions occupying the P and O layers (it is arbitrary to 

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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). 

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