Structures Involving P, T, and O Layers

In Chapter 4 we considered 12 molecular crystal structures based on hep (2P) and ccp (3P) structures. As for metals, many molecular crystal structures are based on bcc (3 2PTOT) structures. 

The B5H9 molecules are square pyramidal. Each boron has one terminal FI and there are four bridging H atoms in the edges of the square 

The molecular structure of S4N4 is unusual, having a tetrahedral arrangement of S atoms with the N atoms at the comers of a square. 

XeF4 is square planar as expected for a molecule with four electron bonding pairs and two unshared electron pairs on Xe. Crystals of XeF4 are monoclinic (P2i/n) with two molecules per cells (a = 5.05, b = 5.92, c = 5.77 A, and (3 = 99.6°). It is a body-centered cell and the notation is 3 2PTOT(t). 

The Body-Centered Crystal Structure of Bis(salicylaldehydato) copper(II) 

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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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