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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slash StartRoot 2 EndRoot"/>. The main interatomic distances involving cations in T/O sites in an fcc/ccp anion array are shown in Fig. 1.25(c) and are summarised in Table 1.11 for the important structure types. These may be used together with the tables of unit cell dimensions, Table 1.9, etc., for calculations on specific compounds. Typical values of bond distances (to oxygen and fluorine) for all the elements are given in Appendix F, where relevant values for different coordination numbers and oxidation states are also given.

      Consideration of atom arrangements in the three structure types described above shows that the concept of cp anions with cations in interstitial sites begins to break down in the fluorite structure. Thus, while the antifluorite structure of Na2O may be regarded as containing ccp O2– ions with Na+ ions in tetrahedral sites, in the fluorite structure of CaF2, it is necessary to regard the Ca2+ ions as forming the ccp array with F ions in tetrahedral sites. In CaF2 the Ca2+ ions have a eutactic ccp arrangement, but are well separated from each other; from Table 1.9 and Table 1.10, Ca–Ca = 3.86 Å, which is much larger than the diameter of a Ca2+ ion (depending on which table of ionic radii is consulted, the diameter of Ca2+ is in the range ~2.2–2.6 Å).

      The F–F distance in CaF2 is 2.73 Å, which indicates that the fluorines are approximately contacting left-parenthesis r Subscript normal upper F Sub Superscript minus Subscript Baseline equals 1.2 minus 1.4 modifying above upper A with ring right-parenthesis. Although the array of F ions in CaF2 is not cp but is primitive cubic, this is perhaps a more realistic way of describing the structure than as containing a eutactic ccp array of Ca2+ ions; nevertheless, both descriptions are used widely.

      1.17.2 Diamond

      This structure type, so important to the semiconductor industry, has already been described, as the zinc blende or sphalerite structure, Fig. 1.29 and Fig. 1.33. The diamond structure is obtained when the two elements in zinc blende are identical, as in diamond. It may therefore be described as a ccp array of carbon atoms, with one set of tetrahedral sites (either T+ or T) occupied also by carbon atoms. It is, however, rather artificial to make a distinction between packing and interstitial atoms since structurally they are identical. Most Group IV elements crystallise with the diamond structure, Table 1.9.

       Table 1.11 Calculation of interatomic distances in some simple structures


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Structure type Distance Number of such distances Magnitude of distance in terms of unit cell dimensions
Rock salt (cubic) Na–Cl 6 a/2 = 0.5a
Cl–Cl 12 a slash StartRoot 2 EndRoot equals 0.707 a
Na–Na 12 a slash StartRoot 2 EndRoot equals 0.707 a
Zinc blende (cubic) Zn–S 4 a StartRoot 3 EndRoot slash 4 equals 0.433 a
Zn–Zn 12 a slash StartRoot 2 EndRoot equals 0.707 a
S–S 12 a slash StartRoot 2 EndRoot equals 0.707 a
Fluorite (cubic) Ca–F 8 a StartRoot 3 EndRoot slash 4 equals 0.433 a
Ca–Ca 12 a slash StartRoot 2 EndRoot equals 0.707 a
F–F 6 a/2 = 0.5a
Zn–S 4 a StartRoot 3 EndRoot slash 8 equals 0.612 a equals 3 c slash 8 equals 0.375 c
Zn–Zn 12 a = 0.612c
S–S 12 a = 0.612c
Nickel arsenidea (hexagonal) Ni–As 6 a slash StartRoot 2 EndRoot equals 0.707 a equals 0.433 c
As–As 12 a = 0.612c
Ni–Ni 2 c/2 = 0.5c = 0.816a
Ni–Ni 6 a = 0.612c
Caesium chloride (cubic) Cs–Cl 8 a StartRoot 3 EndRoot slash 2 equals 0.866 a
Cs–Cs 6 a