ABSTRACT
Basic knowledge concerning the geometric structure of the surface of a solid can be obtained from consideration of its crystal structure. Most frequently the models of the surface are obtained by assuming that the crystal has been cleft along a given crystal plane. In the case of an oxide of sodium chloride structure, e. g., magnesium oxide, cleaving along (100) and (110) planes results in the exposition of surfaces containing equal numbers of cations and anions and hence electrically neutral (Fig. 3.1a and b). However, in the case of the (111) plane, which consists alternatively of cationic one anionic layers, the electrical neutrality of both parts of the split crystal is only reached if, for example, the oxide anions from an anionic plane are equally distributed between the surfaces being formed. Such surfaces will contain half of the unoccupied anionic sites (Fig. 3.1c). The ions on the planes created by cleaving the crystal are coordinatively unsaturated. The coordination number of a cation (and also of an anion) in the (100) face of MgO is 5 instead of S in the bulk. On the (110) it is reduced to 4 and on (111) to 3, but depending on the distribution of anions in their most external half-filled plane it might also be 4 or 5.
