Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 52

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

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
	Na–Na	12
Zinc blende (cubic)	Zn–S	4
	Zn–Zn	12
	S–S	12
Fluorite (cubic)	Ca–F	8
	Ca–Ca	12
	F–F	6	a/2 = 0.5a
Wurtzitea (hexagonal)	Zn–S	4
	Zn–Zn	12	a = 0.612c
	S–S	12	a = 0.612c
Nickel arsenidea (hexagonal)	Ni–As	6
	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
	Cs–Cs	6	a
	Cl–Cl	6	a
Cadmium iodide (hexagonal)	Cd–I	6
	I–I	12	a = 0.612c
	Cd–Cd	6	a = 0.612c

^a These formulae do not necessarily apply when c/a is different from the ideal value of 1.633.