Structures with Multiple Layers

In earlier chapters we dealt with crystal structures of elements and molecules involving close packing of P layers, compounds involving P and O layers or P and T layers, and those with combinations of P, O, and T layers. The body-centered cubic (bcc) structure is common for metals, and molecular crystals, and the related CsCl structure is common for ionic compounds. It has been shown that the bcc structure can be included in the PTOT system with all P, T, and O sites occupied. In this chapter we examine cases that are variations of the regular PTOT pattern. Nature’s preference for the regular PTOT pattern seems to be based on efficiency of packing, high symmetry, and high coordination. Deviations occur for chemical bonding not well adapted to the regular PTOT pattern and for unusual sizes or shapes of building units. In some cases large groups of atoms, molecules, or ions are the repeating units. Some structures included here contain double layers consisting of puckered layers or a layer consisting of two packing positions. A few structures included have triple layers.