It is usual for defects in ceramic materials to be described using a short hand notation after Kroger and Vink.6 In this, the defect is described by its chemical formula. Thus, a sodium ion would be described as Na, whatever its position in whatever lattice. A vacancy is designated as ‘V.’ The description is made with respect to the position within the lattice that the defect occupies. For example, a vacant Mg site is designated by VMg and an Na substituted at an Mg site is designated by NaMg. Interstitial ions are represented by ‘i’ so that an interstitial fluorine ion in any lattice would be Fi.

The charge on an ion is described with respect to the site that the ion occupies. Thus, an Na ion (which has formal charge +) sitting on an Mg site in MgO (which expects to be occupied by a 2+ ion) has one too few + charges; it has a relative charge of 1 — which is designated as a vertical dash, meaning that it is written as NaMg. An Al3+ ion at an Mg site in

MgO has too high a charge. Positive excess charge relative to a site is designated with a dot, thusAlMg. Similarly, a vacant Mg site in MgO is designated by VMg and an interstitial Mg ion in MgO byMg**. Finally, a neutral charge is indicated by a cross ‘x,’ so that an Mg ion at an Mg site in MgO is MgMg.

Ions such as Fe may assume more than one oxida­tion state. Therefore, in MgO, we might find both Fe2+ and Fe3+ ions on Mg sites, that is, FeMg andFeMg. It is also possible to encounter bound defect pairs or clus­ters. These are indicated using brackets and an indica­tion of the overall cluster charge; for example, an Fe3+ ion bound to an Na+ ion, both substituted at magne­sium sites, would be {FeMg: NaMg| . These cases are summarized in Figure 5.

Finally, defect concentrations are indicated using square brackets. Thus, the concentration of Fe3+ ions substituted at magnesium sites in MgO would be
[FeMg] . When we consider the role of hole and elec­tron species, these are represented as h* and e0, respectively.