Solid State Chemistry and its Applications. Anthony R. West. Читать онлайн. Newlib. NEWLIB.NET

Автор: Anthony R. West
Издательство: John Wiley & Sons Limited
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Жанр произведения: Химия
Год издания: 0
isbn: 9781118695579
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1.641 GaN 3.180 5.166 1.625 ZnTe 4.27 6.99 1.637 InN 3.533 5.693 1.611 BeO 2.698 4.380 0.378 1.623 TaN 3.05 4.94 1.620 CdS 4.1348 6.7490 1.632 NH4F 4.39 7.02 0.365 1.600 CdSe 4.30 7.02 1.633 SiC 3.076 5.048 1.641 MnS 3.976 6.432 1.618 MnSe 4.12 6.72 1.631

      R. W. G. Wyckoff, Crystal Structures, Vols 1 to 6, Wiley (1971).

      The three anions at c equals one half that form the base of this T site also form the base of a T+ site shown in Fig. 1.35(e) centred at 0 comma 0 comma five eighths. The apex of this tetrahedron is the anion at the top corner with coordinates 0, 0, 1. Another T+ site at one third comma two thirds comma one eighth is coordinated to three anions in the basal plane and an anion at one third comma two thirds comma one half (d). The triangular base of this site, at c = 0, is shared with a T site underneath (not shown) at one third comma two thirds comma negative one eighth. The equivalent T site that lies inside the unit cell is at one third comma two thirds comma seven eighths (e).

      The octahedral site in Fig. 1.35(d) is coordinated to three anions at c = 0 and three anions at c equals one half. The centre of gravity of the octahedron lies midway between these two groups of anions and has coordinates two thirds comma one third comma one quarter. The second octahedral site lies immediately above the octahedral site shown in (d) and has coordinates two thirds comma one third comma three quarters (e). The three anions at c equals one half are therefore common to the two octahedra, which means that octahedral sites share opposite faces.

      The coordination environments of the cations in wurtzite and NiAs are emphasised in Fig. 1.35(f) and (g). Zinc is shown in T+ sites and forms ZnS4 tetrahedra (f), linked at their corners to form a 3D network, as in (j). A similar structure results on considering the tetrahedra formed by four Zn atoms around a S. The tetrahedral environment of S (1) is shown in (f). The SZn4 tetrahedron which it forms points down, in contrast to the ZnS4 tetrahedra, all of which point up; on turning the SZn4 tetrahedra upside down, however, the same structure results.

      Comparing larger scale models of zinc blende [Fig. 1.33(b)] and wurtzite [Fig. 1.35(j)], they are clearly very similar and both can be regarded as networks of tetrahedra. In zinc blende, layers of tetrahedra form an ABC stacking sequence and the orientation of the tetrahedra within each layer is identical. In wurtzite, the layers form an AB sequence and alternate layers are rotated by 180° about c relative to each other.

      The NiAs6 octahedra in NiAs are shown in Fig. 1.35(g). They share one pair of opposite faces (e.g. the face formed by arsenic ions 1, 2 and 3) to form chains of face‐sharing octahedra that run parallel to c. In the ab plane, however, the octahedra share only edges: As atoms 3 and 4 are shared between two octahedra such that chains of edge‐sharing octahedra form parallel to b. Similarly, chains of edge‐sharing octahedra form parallel to a (not shown). A more extended view of the octahedra and their linkages is shown in (k).

      The NiAs structure may also be regarded as built of AsNi6 trigonal prisms, therefore, which link up by sharing edges to form a 3D array. In Fig. 1.35(i), each triangle represents a prism in projection down c. The prism edges that run parallel to c, i.e. those formed by Ni at c equals one quarter and three quarters in (h), are shared between three prisms. Prism edges that lie in the ab plane are shared between only two prisms, however. In (i), the edge xy is shared between As at c equals one half and c = 0. The structure therefore has layers of prisms arranged in an … ABABA … hexagonal stacking sequence, as shown further in (1).

      The NiAs structure can be described as hcp As with Ni in fully occupied octahedral interstitial sites. However, unlike the case of NaCl where Na and Cl positions are interchangeable, we cannot simply exchange Ni, As and arrive at the same structural description. If we consider the arrangement of Ni alone, it still forms cp layers but the stacking sequence along c is identical because Ni atoms are superposed in projection, Fig. 1.35(h). Since both As and Ni form cp layers, we