Inorganic Solids

The cubic closest packing (ccp) or face-centered cubic packing (fcc) of spheres used as models for crystal structures not only enable us to visualize the metal structures, this model correlate the structures of NaCl, CaF₂, zinc-blende, and Li₃Bi structures very well. Each of these is a structure type common to many other compounds.

In this diagram, the ccp packing is shown in the center. When Cl- ions form an array of ccp packing, and the octahedral sites occupied by Na+ ions, we have the NaCl structure. Of course, in this case, the sodium ions are smaller than the chloride ions even though they have the same number of electrons. The smaller size is due to a positive charge on the sodium ion.

The radii for Ca2+ and F- ions are 99 and 71 pm respectively. Thus, it is fair to consider the larger Ca2+ ions forming a close packed array, with the F- ions filling all the tetrahedral sites. Since there are twice as many tetrahedral sites, the stoichiometry is CaF2. In this structure, the ratio of anion radius to cation radius is 0.71, and the calcium ions have a coordination number 8 according to the radius ratio rules. Thus, the calcium ions are pushed far apart in terms of packing, reducing their repulsion. Using the sphere packing geometry helps us to recognize the arrangement of the ions in space.

The material used for fission nuclear reactor is a uranium oxide, UO2, and it has the same structure as CaF2. The uranium cations are larger than the oxygen anions.

In an anti-fluorite structure, the anions occupy the Ca2+ position whereas the cations occupy the F- positions. An example of antifluorite structure is Mg2Sn, in which Sn is surrounded by 8 a Mg cube with Sn-Mg distances of 293 pm, and the Mg is surrounded by 4 Sn in a tetrahedral fashion. Heyes mentioned Na2O as having an anti-fluorite structure.

The zinc-blende structure can be viewed as a ccp array of S atoms with Zn occupying half of the tetrahedral sites, resulting in a ZnS stoichiometry. In this compound, there is considerable Zn-S covalent bonding, and the structure is actually very similar to that of diamond. Yet, since the Zn-S bonding is weak, zinc-blende is much softer. Note however, that the covalent radii of zinc and sulfur are 133 and 103 respectively.

The Li3Bi structure is very interesting in that it may be seen as a result of occupying all the octahedral and tetrahedral sites by Li in the ccp array of Bi atoms. To call this an ionic solid consisting of Li+ and Bi3- ions is somewhat drastic, but Bi is in the same group as N, P, As, and Sb.


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Inorganic Solids 2010-04-11T07:47:00+07:00 Rating: 4.5 Diposkan Oleh: andhiart ab