Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 47

To summarise, the rock salt structure has ccp/fcc anions with octahedral sites fully occupied by cations and tetrahedral sites empty. Each cation is surrounded by six anions, as shown for cations 1, 2 and 3 in Fig. 1.31; similarly, each anion is octahedrally coordinated by cations (to see this, consider the anion at the top face centre, ½½1, which is coordinated to the four edge centre cations in the top face, to the cation at the body centre, shown, and to a sixth cation at the body centre of the unit cell above).

The (NaCl₆) or (ClNa₆) octahedra share common edges, Fig. 1.31. Since each has 12 edges and each edge is common to two octahedra, it is difficult to represent this satisfactorily in a drawing; Fig. 1.31 shows just two such linkages. A perspective which focuses on the 3D array of octahedra is shown in Fig. 1.32; the structure may be regarded as layers of octahedra, which also have an ABC stacking sequence similar to the anions. Each octahedron face is parallel to a cp layer of anions, as emphasised by the numbering or shading of coplanar faces. Parts of four different sets of faces are seen, corresponding to the four cp orientations in a ccp/fcc array. Also shown, arrowed, in Fig. 1.32 are the empty tetrahedral sites.

A large number of AB compounds possess the rock salt structure. A selection is given in Table 1.8 together with values of the a dimension of the cubic unit cell. Most halides and hydrides of the alkali metals and Ag⁺ have this structure, as do a large number of chalcogenides (oxides, sulphides, etc.) of divalent metals. Many are ionic but others are either metallic, e.g. TiO, or covalent, e.g. TiC.

Figure 1.31 Unit cell of the rock salt structure showing edge‐sharing octahedra.

Figure 1.32 The rock salt structure as an array of edge‐sharing octahedra.