In binary packing complexes, there is no preferential bonding between types of component — either between like or between unlike components. A large variety of phase diagrams and structural types are found, and there are resemblances to metal-alloy systems. Correlation of phase diagrams and crystal structures can be considerably hampered by the occurrence of disorder and conformational isomerism. When the components resemble each other chemically and geometrically, extensive solid solution can occur with possibilities of order and/or disorder. A few solid solutions of the interstitial type are known. These form especially when the component molecules differ appreciably in size, and are not always clearly distinguishable from some clathrates. The best defined group is that analogous to intermetallic compounds. Here, the molecular complex forms a phase different from those of its components, although echoes of structural features of one or both components often appear in the structure of the molecular complex, then dubbed ‘mimetic’. Molecular complexes of the fullerenes (C60, C70, and C76) are among the examples of ‘ordered packing complexes’ discussed. Crystals containing ‘solvent of crystallization’ are included in this group when there is no preferential host-host or host-guest interaction.
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