Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 54

The unit cell of CsCl is shown in Fig. 1.36. It is a primitive cube, containing Cl at corners and Cs at the body centre, or vice versa (note that it is not body centred cubic since there are different ions at corner and body centre positions). The coordination numbers of both Cs and Cl are eight with interatomic distances of 0.866a, Table 1.11. The CsCl structure is not cp. In a cp structure, each anion has 12 other anions as next nearest neighbours whereas in CsCl, each Cl has only six Cl^– ions as next nearest neighbours (arranged octahedrally). Some compounds with the CsCl structure are given in Table 1.14. They fall into two groups, halides of large monovalent elements and a variety of intermetallic compounds.

Lanthanum hexaboride, LaB₆, has the CsCl structure in which Cl is replaced by the octahedral cluster anion B₆ ³⁻. LaB₆ has a low work function (i.e. it ionises easily) of 2.5 eV, very high electron emissivity, metallic conductivity and is stable in vacuum. These properties enable it to be used as a hot cathode in the electron gun in electron microscopes: advantages are that it has higher brightness and longer lifetime than traditional tungsten filament cathodes.

Although CsCl is not a cp structure, there is a link between it and the fluorite structure, which can be described as a primitive cubic array of anions with cations in alternate cube body centres, Fig. 1.34; in CsCl, all body centres are occupied.

Table 1.14 Some compounds with the CsCl structure

Compound	a/Å	Compound	a/Å	Compound	a/Å	Compound	a/Å
CsCl	4.123	NH4Br	4.0594	CuPd	2.988	AlNi	2.881
CsBr	4.286	TlCl	3.8340	AuMg	3.259	LiHg	3.287
CsI	4.5667	TlBr	3.97	AuZn	3.19	MgSr	3.900
CsCN	4.25	TlI	4.198	AgZn	3.156
NH4Cl	3.8756	CuZn	2.945	LiAg	3.168