Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 43

Since cp structures provide an efficient means of packing atoms, many molecular compounds crystallise as cp structures even though the bonding forces between adjacent molecules are weak van der Waals forces. If the molecules are roughly spherical or become spherical because they can rotate or occupy different orientations at random, then simple hcp or ccp structures result, e.g. in crystalline H₂, CH₄ and HCl. Non‐spherical molecules, especially if they are built of tetrahedra and octahedra, can also fit into a cp arrangement. For example, Al₂Br₆ is a dimer with two AlBr₄ tetrahedra sharing a common edge, Fig. 1.26(a). In crystalline Al₂Br₆, the Br atoms form an hcp array and Al atoms occupy one‐sixth of the available tetrahedral sites. One molecule, with the Br atoms in heavy outline is shown in Fig. 1.26(b); Al atoms occupy one pair of adjacent T₊ and T_– sites. Br atoms 3 and 5 are common to both tetrahedra and are the bridging atoms in (a). Adjacent Al₂Br₆ molecules are arranged so that each Br in the hcp array belongs to only one molecule. SnBr₄ is a tetrahedral molecule and also crystallises with an hcp Br array, but only one‐eighth of the tetrahedral sites are occupied.

Figure 1.26 (a) hcp arrangement of Br atoms in crystalline Al2Br6; (b) Al atoms occupy T+ and T– sites. Dashed circles are below the plane of the paper.