Crystal Structures of Halides, Oxides, Chalcogenides, Pnictides

1 Crystal Structures of Halides, Oxides, Chalcogenides, Pnictides 

Square coordination of atoms exists in CuO, SnO, PdO and PtO, where M-0 bond distances equal 1.954, 2.22, 2.01 and 2.02 A, respectively. In PbO the lead 

In Table 5.4 there are listed interatomic distances in MX solids of the structure type NaCl (Bl), where atoms have octahedral coordination. These compounds contain metal atoms with low ENs, forming bonds of an essentially ionic character. Therefore compounds with the B1 structure are usually considered as typically ionic. Comparison of interatomic distances in structures of the B3 and Bl types (Tables 5.3 and 5.4) shows that such increase in coordination of atoms is accompanied by an increase of the bond lengths by a factor of 1.080(9). Interatomic distances in crystal of this type are additive. Differences of the bond lengths in halides MX are Afi Na-Li = (f(Na—X) — (f(Li—X) = 0.28 A, Ai K-Na = 0.34 A, Ai Rb-K = 0.015 A, Afi Cs-Rb = 0.18 A, A /cs-NH4 = 0-17 A, A /NH4 Ag = 0.52 A, A fxi-Ag = 0.41 A. This principle works very well because of similar character of chemical bonds (for example, for halides K, Rb and Cs the deviation is ca. 5 %). On the contrary, if we compare hydrides and fluorides of alkali metals where the bond character is different, we get Ad = /(M-H)- /(M-F) = 0.15 A 35 %. In the case of oxides and chalco-genides of the MX type, the additive principle is correct within 8 %. Comparison of data in Tables 3.2, S3.1 and 5.4 shows, that the ratio of the bond lengths for = 1 to 

Crystals of the structure type CsCl (B2) have cubic coordination of atoms (Table 5.5). Atoms of metals in these compounds are large, with low ENs and hence they form the most ionic bonds. The average ratios d(Nc — 8)ld(Nc = 1) for the substances listed in Table 5.5, is equal to 1.26(5). 

Crystals of the MX2 type (Fig. 5.5) have coordination numbers from 2 up to 12. Usually as examples of crystals with Nc = 2, halides of mercury are chosen which are considered as molecular substances. However, actually, in the structure type HgCb the array of atoms resembles the structure of PbCb but with shorter distances between Hg and two nearest neighbors. Therefore = 2 in this case only formally the problem of distorted coordination polyhedra and effective coordination numbers will be discussed later. Twofold coordination of the metal atoms is found in structures 

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