Structures Based on Linked Polyhedra

Directional covalent bonding. hydrogen bonding and special electrostatic effects may lead to cation coordination preferences not expected on size grounds, and to more open structures for which the description in terms of anion packing is not very useful. Some of these structures are conveniently described in terms of linked polyhedra or groups of polyhedra. It is also possible, and indeed informative, to describe close packed ionic structures in terms of linked polyhedra and this is studied first .

Linkage of polyhedra in close packed ionic structures

First, inspect a two dimensional layer of octahedra of the type shown in Figure 11.

  1. Compare the relative positions of the octahedral vertices at the top and bottom of a layer with the relative positions of the anions in the AB packing you studied earlier.
  2. Verify that the two models are equivalent representations.
  3. By drawing arrange a pair of layers to represent the hexagonal close packed structure (hcp) and another pair to represent cubic close packing (ccp).
  4. Add a third layer of octahedra to each structure to continue the three dimensional array.
  5. Study the location of the tetrahedral holes in the two models you have built (insert loose tetrahedra if you wish) and observe how hcp and ccp arrays can be constructed from linked tetrahedra.
  6. Now consider your unknown structure from the ions section and if necessary enlarge your model of structure by completing the appropriate anion coordination of each metal atom.
  7. Why do you think that structures are not observed with face-shared tetrahedra?
  8. Why do you think that NH4Cl has the CsCl structure but NH4F has the wurtzite structure?

Descriptions of open structures in terms of linked polyhedra

Figures 12 , 13, 14, 15, and the corresponding models, illustrate some finite and infinite groups of octahedra and tetrahedra.

  1. In each case determine the stoichiometry implied by the coordination numbers and suggest one compound representing each structural type.
  2. Sketch the fragments.

Now consider the two extended structures and corresponding models composed of linked polyhedra that are shown in Figures 16 and 17.

Sketch the structures and answer the following questions:

  1. Give the coordination numbers and deduce the overall stoichiometry?
  2. Identify the structural type and give a further example of a compound adopting the structure.

Displays of the structures of many of the materials examined in this practical are available for inspection.

Note also that there is a fine exhibition of minerals in the neighbouring University Museum.