Non-close-packed lattices

The structures of the interstitial systems, as their name implies, can all be described in geometrical terms as an array of metal atoms with the smaller atoms of the non-metallic element disposed in the interstices, and it is found that they are formed only if the ratio of the radii of the atoms is less than about 0 59. The arrangement of the metal atoms alone is usually that of one of the three structures common to the true metals, i.e. cubic or hexagonal close packed with a co-ordination number 12 or cubic body centred with a co-ordination number 8, but occasionally these atoms are arranged on a simple hexagonal lattice. 

Therefore lattice structures with closely packed planes allow more plastic deformation than those that are not closely packed. Also, cubic lattice structures allow slippage to occur more easily than non-cubic lattices. This is because of their symmetry which provides closely packed planes in several directions. Most metals are made of the body-centered cubic (BCC), face-centered cubic (FCC), or hexagonal close-packed (HCP) crystals, discussed in more detail in the Module 1,... 

Magnetite possesses an inverse spinel structure with oxygen ions forming a face-centred cubic closely packed structure. The formula for describing Fe occupancy is (Fe " ) [Fe ", Fe ]04 where the parentheses ( ) stand for cations at tetrahedral sites while brackets [ ] denote cations at octahedral lattice sites. Stoichiometric magnetite has all available substitutional sites occupied by Fe and Fe ions. Non-stoichiometric magnetites also exist, with various numbers of available sites being either vacant or occupied by impurity ions. 

---