Halides cubic structure

In general, the cadmium halides show in their crystal structure the relation between polarizing effect and si/e of anion. The tluoride has tile smallest and least polarizable anion of Ihe lour and forms a cubic structure, while the mure polarizable heavy halides have hexagonal layer structures, increasingly covalent and al increasing distances apart in inxler down tire periodic table, in solution the halides exhibit anomalous thermal and transport properties, due primarily to the presence of complex ions, such as CDlr and CdBr r. especially in concentrated solutions or those containing excess halide ions. 

Table 7.11 lists all the elements, several oxides, halides, and chalcogenides and the crystal structures of their more usual and stable polymorphs. Figure 7.19 shows a few elementary cubic structures. 

The structure of sodium chloride, which is the prototype for most of the alkali halides, is best described as a cubic closest packed array of Cl- ions with the Na+ ions in all of the octahedral holes [see Fig. 16.42(b)]. The relative sizes of these ions are such that rua 0.66i ci-> so this solid obeys the guidelines given previously. Note that the CP ions are forced apart by the Na+ ions, which are too large for the octahedral holes in the closest packed array of CP ions. Since the number of octahedral holes is the same as the number of packed spheres, all the octahedral holes must be filled with Na+ ions to achieve the required 1 1 stoichiometry. Most other alkali halides also have the sodium chloride structure. In fact, all the halides of lithium, sodium, potassium, and rubidium have this structure. Cesium fluoride has the sodium chloride structure but because of the large size of Cs+ ions, in this case the Cs ions form a cubic closest packed arrangement with the F ions in all the octahedral holes. On the other hand, cesium chloride, in which the Cs+ and CP ions are almost the same size, has a simple cubic structure of CP ions, with each Cs+ ion in the cubic hole in the center of each cube. The compounds cesium bromide and cesium iodide also have this latter structure. 

Finally, 3-dimensional NHg frameworks similar to those in the forms of Si02 occur in Millon s base, NHg20H.xH20 (x=l or 2), and its salts/ The framework in the nitrate, NHg2N03, is of the cristobalite type, and other salts also have this cubic structure if prepared from the nitrate (Fig. 26.8(e)), but it seems likely that the hexagonal (tridymite-like) structure of Fig. 26.8(f) is more stable for the halides. The base itself and the bromide and iodide can be prepared initially in this hexagonal form or by heating the cubic modifications. In these frameworks N forms four tetrahedral bonds and Hg two collinear bonds (Hg-N = 2-07 A). The OH , X or other ions occupy the interstices in the frameworks. 

When the pseudo-spherical ammonium ion is mostly replaced by a truly spherical ion the complex sequence of phase changes found in the pure ammonium halides is suppressed. The mixed potassium ammonium halide salts retain their NaCl cubic structure down to the lowest temperatures. The alkali metal ions support the structure leaving the ammonium ions as free to rotate at 1 K as at 300K [13]. The INS spectrum of this system is quite different from the pure salt and there are no sharp features in any region of the spectrum. We shall analyse the impact that this freedom has on the internal modes about 1400 cm. ... 

Yet a third simple cubic structure, not found among the alkali halides, occurs nevertheless in a limited number of ionic compounds of composition AX. This is the zincblende arrangement, shown in fig. 3.04, in which the co-ordination is fourfold or tetrahedral, each ion being... 

The clathrates G AgXg (A = P, As, or Sb X = Cl, Br, or I) are synthesized in crystalline form either from the elements or from germanium and suitable halides. At temperatures between 600 and 700 C, they decompose to the elements, halides, and other unidentified products. The cubic structure consists of a three-dimensional E43 framework of tetrahedrally bound 38 Ge and 8 A atoms the 8X atoms are situated inside the big cages of this frame. ... 

Cesium halides differ from other alkali halides crystallographically. They are simple cubic while the other alkali halides have a face-centered cubic structure. Because of this, R ions occupy the interstitial sites in cesiiun halides instead of the substitutional sites in the other alkali halides. 

It has since been established that of the 20 alkali metal halides, MX, seventeen have face-centered cubic structures. The exceptions, CsCl, CsBr, and Csl, aU have body-centered cubic structures where each ion is surrounded by eight counterions at the comers of a cube. 

The summation is over the different types of ion in the unit cell. The summation ca written as an analytical expression, depending upon the lattice structure (the orij Mott-Littleton paper considered the alkali halides, which form simple cubic lattices) evaluated in a manner similar to the Ewald summation this typically involves a summc over the complete lattice from which the explicit sum for the inner region is subtractec... 

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