Lattice packings

Crystal lattice packing, 12 249-250 Crystal lattice vibrations, 14 236 Crystalline adsorbents, 1 586, 589. See also Molecular sieves Zeolites for gas separation, 1 631 properties and applications, l 588t Crystalline alkali silicates, atomic structure of, 22 454-455 Crystalline cellulose, 5 373-379 Crystalline epoxy resins, 10 373-374 Crystalline flake graphite, 12 793 manufacture and processing of, 12 781-784... 

Not all structures are based on close packed lattices. Ions that are large and soft often adopt structures based on a primitive or body centred cubic lattice as found in CsCl (22173) and a-AgI (200108). Others, such as perovskite, ABO3 (Fig. 10.4), are based on close packed lattices that comprise both anions and large cations. The larger and softer the ions, the more variations appear, but the lattice packing principle can still be used. Santoro et al. (1999,2000) have shown how close-packing considerations combined with the use of bond valences can give a quantitative prediction of the structure of BaRuOs (10253). 

A related phenomenon can also occur when the crystal lattice packing is chiral. This intrinsic handedness can result in formation of a 1 1 mixture of enantiomeric crystals. In this case, although there has been self-resolution into (+)- and (—)-crystals, both molecular enantiomers remain unseparated in each crystal. The fundamental distinction is that a conglomerate single crystal contains only one molecular enantiomer and therefore would be optically active in solution, while, for the latter, a single crystal contains both molecular enantiomers and its solution would be optically inactive. 

Figure 14 Structural representation of lattice packing for I-III-VI2 materials...

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