Bond length alkali halides

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... 

The Shannon-Prewitt ionic radii (r+ + r ) are based on the most ionic compounds, the fluorides and oxides for the radii of the metal cations, and the alkali hahdes for the radii of the anions of the remaining halides. The shortening of silver halide bond lengths is attributahle to polarization and covalency. 

Although the simple model does give a semiquantitative account of bond length, cohesion, and compressibility for KCl, it is less useful to list the results of doing the arithmetic for the other alkali halides than merely to list the experimental parameters, as in Table 13-4. Predictions from the simple model will be approximately as accurate as the KCl results are, and more accurate models can be fitted to the experimental parameters if one wishes. Extension of the model to ions... 

The most remarkable aspect of Eq. (14-10) is that the dependence upon bond length and therefore, the dependence upon which alkali halide is being considered—has cancelled out. Another consequence is that the ion. softening does not depend upon pre.ssurc. This explains why theories of the crystal-field splitting and its pressure dependence that arc based upon hard ions have been successful the 0.51 factor can be absorbed in an undetermined scale parameter, depending upon the shape of the orbitals being split, since the factor 0.51 does not change with distortion. 

In this connection it is interesting to note that the difference between pairs of bond lengths M-F and M-Cl is approximately equal in many cases to the difference between the ionic radii (0-45 A) rather than to the difference between the covalent radii (0 27 A) of F and Cl. This is to be expected for ionic crystals and molecules (for example, gaseous alkali-halide molecules) but it is also true for the following molecules ... 

Ammonium fluoride, NH4F, crystallizes with a structure different from those of the other ammonium (and alkali) halides. The chloride, bromide, and iodide have the CsCl structure at temperatures below 184 3°, 137-8 , and — IT S C respectively, and the NaCl structure at temperatures above these transition points, but NH4F crystallizes with the wurtzite structure, in which each N atom forms N-H—F bonds of length 2-71 to its four neighbours arranged tetrahedrally around it. This is essentially the same structure as that of ordinary ice. 

Raman spectroscopy can detect changes in the C-C bond length, since the RBM varies with diameter and the G band varies with the axial C-C bond length. Moreover, the band intensity also varies as charge transfer occurs, either to or from the nanotubes. Gupta et al [84] monitored the dependence of the C-C bond length in an SWCNT material on charge transfer, for several alkali, halide, and sulfate... 

Sanderson RT (1952) An interpretation of bond lengths in alkali halide gas molecules. J Am... 

For strongly ionic compounds such as the alkali halides the closed-shell or rare gas approximation to the electronic structure of the ions is extremely accurate. Goldschmidt and Pauling have shown that for such compounds there exist strong correlations between many physical properties (crystal structure, bond lengths, compressibilities, heats of fusion and sublimation, melting and boiling points, solubility) and the ratio p = R of cation univalent radius to anion... 

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