Crystal packing stoichiometry

Fig. 4 Crystal packing diagrams of a 17 16 and b 18 16, both with 2 1 stoichiometry, and c 19 16 benzene with 2 1 1 stoichiometry (dashed lines represent hydrogen bonds and close contacts) [43]...

Of course, more complicated situations may exist, such as n m stoichiometries or ternary salts. See, for example, Graja [142] for a classification of molecular conductors on the basis of conductivity, crystal packing, and charge transfer. See also Pouget [114] for a discussion of charge transfer and instabilities in a number of materials. 

Gavezzotti, A. Crystal packing of hydrocarbons. Effects of molecular size, shape and stoichiometry. Aeta Crystallogr.,... 

In recent years, ionic assemblies have received significant attention for [2 -I- 2] cycloaddition reaction in the solid state for several reasons greater stability due to robust and directional charge assisted H-bonded ionic heterosynthons over their neutral assemblies combination of more than one cooperative non covalent interaction in order to stabilise crystal packing, better control of the stoichiometry better solubility and easy separation of the photoproduct from the ionic auxiliary template. 

I shall take the simple view that most metal oxide structures are derivatives of a closest packed 02 lattice with the metal ions occupying tetrahedral or octahedral holes in a manner which is principally determined by size, charge (and hence stoichiometry) and d configuration (Jj). The presence of d electrons can lead to pronounced crystal field effects or metal-metal bonding. The latter can lead to clustering of metal atoms within the lattice with large distortions from idealized (ionic) geometries. 

Many simple minerals, especially simple salts like halite, NaCl, sulfides, sulfosalts and oxides, have structures based upon cubic or hexagonal closest-packed arrays of either cations or anions. Coordination geometries of metal ions in many of these kinds of minerals are thus confined to more or less regular octahedra and tetrahedra. The occupancy of the two types of sites is dictated by the stoichiometry of the mineral, the radius of the ions involved and their preferred coordination geometries. Coordination of cations in mineral species in terms of bonding and crystal field effects has been extensively reviewed.16-21 Comprehensive lists of ionic radii relevant to cation coordination geometries in minerals have also been compiled.16,21... 

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