IONIC CRYSTAL STRUCTURES

General Bonding considerations.
For compound (A)n(B)m we can expect ionic bonding to predominate when atom A has low electronegativity and atom B has a high electronegativity. In this case electron transfer from one atom to another leads to the formation of A+B-. For the main group elements the electron transfer continues until the ions have closed shell configurations.

For ionic compounds the bonding forces are electrostatic and therefore omni-directional. The bonding forces should be maximized by packing as many cations around each anion, and as many cations around each anion as is possible. The number of nearest neighbor ions of opposite charge is called the coordination number. We must realize however that the coordination numbers are constrained by the stoichiometry of the compound and by the sizes of the atoms.

e.g. For sodium chloride, Na+Cl-, there are 6 anions around each cation (coordination number Na = 6); because of the 1:1 stoichiometry there must also be 6 Na cations around each Cl anion. For Zr4+O2-2 there are 8 anions around each cation, therefore there must be only 4 cations around each anion.

Simple ionic crystal structures can be approached in terms of the close packing procedures developed for metallic structures. In most (but by no means all) ionic compounds the anions are larger than the cations. In these cases it is possible to visualize the structures in terms of a close packed arrangement of the larger anions, with the cations occupying the vacant interstices between the close packed layers. Recall that although ccp & hcp are the most efficient ways of packing spheres, only 74% of the available space is filled, the 26% "free space" is in the form of different types of holes or sites which can be occupied by the smaller cations in the ionic structures .

First let us consider the types of holes available in a close packed anion arrangement.


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