Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 31

The unit cell of a ccp arrangement is the familiar face centred cubic (fcc) unit cell, Fig. 1.11(c), with spheres at corner and face centre positions. The relation between ccp and fcc is not immediately obvious since the faces of the fcc unit cell do not correspond to cp layers. The cp layers are, instead, parallel to the {111} planes of the fcc unit cell. This is shown in Fig. 1.20 and Appendix B. The spheres labelled 2–7 in Fig. 1.20(a) form part of a cp layer, as revealed by removing a corner sphere 1 in (b) and comparing (b) with Fig. 1.16(a). The orientations of (a) and (b) in Fig. 1.20 are the same but the spheres in (b) are shown larger. A similar arrangement to that shown in (b) would be seen on removing any corner sphere in (a) and, therefore, in a ccp structure, cp layers occur in four orientations. These orientations are perpendicular to the body diagonals of the cube (the cube has eight corners but only four body diagonals and, hence, four different orientations of the cp layers). The cp layers in one orientation are seen edge‐on in (c) and in another orientation, perpendicular to the layers in (d); (c) is the same as (a) but rotated slightly; similarly, (d) is the same as (b) but also rotated slightly. In (d), all atoms of the unit cell, spanning four cp layers, ABCA, are shown in projection down a <111> direction.