Halides rock salt structure

Iodide ions reduce Cu to Cu , and attempts to prepare copper(ll) iodide therefore result in the formation of Cul. (In a quite analogous way attempts to prepare copper(ll) cyanide yield CuCN instead.) In fact it is the electronegative fluorine which fails to form a salt with copper(l), the other 3 halides being white insoluble compounds precipitated from aqueous solutions by the reduction of the Cu halide. By contrast, silver(l) provides (for the only time in this triad) 4 well-characterized halides. All except Agl have the rock-salt structure (p. 242). Increasing covalency from chloride to iodide is reflected in the deepening colour white yellow, as the... 

The saline hydrides are white, high-melting-point solids with crystal structures that resemble those of the corresponding halides. The alkali metal hydrides, for instance, have the rock-salt structure (Fig. 5.39). 

Complex lithium halide spinels (Kanno, Takeda and Yamamoto, 1982 Lutz, Schmidt and Haeuseler, 1981), based on Li2CdCl4 and Li2MgCl4 have remarkably high Li ion conductivity for close packed structures. Fig. 2.11. These are complicated materials however, they have essentially inverse spinel structures but may exist also in various distorted forms. Some of them undergo a phase transition to defect rock salt structures at high temperatures some are non-stoichiometric. 

If we take a series of alkali metal halides, all with the rock salt structure, as we replace one metal ion with another, say sodium with potassium, we would expect the metal-halide internuclear distance to change by the same amount each time if the concept of an ion as a hard sphere with a particular radius holds true. Table 1.8 presents the results of this procedure for a range of alkali halides, and the change in internuclear distance on swapping one ion for another is highlighted. 

Cadmium is a member of Group 12 (Zn, Cd, Hg) of the Periodic Table, having a filled d shell of electrons 4valence state of +2. In rare instances the +1 oxidation state may be produced in the form of dimeric Cd2+2 species [59458-73-0], eg, as dark red melts of Cd° dissolved in molten cadmium halides or as diamagnetic yellow solids such as (Cd2)2+ (AlCl [79110-87-5] (2). The Cd + species is unstable in water or other donor solvents, immediately disproportionating to Cd2+ and Cd. In general, cadmium compounds exhibit properties similar to the corresponding zinc compounds. Compounds and properties are listed in Table 1. Cadmium(TT) [22537 48-0] tends to favor tetrahedral coordination in its compounds, particularly in solution as complexes, eg, tetraamminecadmium(II) [18373-05-2], Cd(NH3)2+4. However, solid-state cadmium-containing oxide or halide materials frequently exhibit octahedral coordination at the Cd2+ ion, eg, the rock-salt structure found for CdO. 

A. Oxides and Halides with Rock Salt Structure... 

IV. Oxides and Halides with the Rock Salt Structure Surface Structure, Reactivity, and Catalytic Activity... 

The internuclear distance in the crystal, Roj is 2.88 A., and the covalent radius sum Rc is 2.48. The Ag—Ag bond energy has been estimated, from data for all the silver halides, to be about 19 kcal. per mole. The dissociation energy of Br2 is 46.1 kcal. per mole, from which the geometric mean for AgBr is found to be 29.5. The six bonds that must be broken for atomization, per formula unit, of this rock salt structure utilize only four electron pairs, from which n = 4. The covalent contribution is ... 

All the alkali metal halides except the cliloride, bromide and iodide of caesium form cubic crystals with the rock salt lattice and show a co-ordination number of 6. The exceptions are also cubic, but have the caesium chloride structure (Fig. 133) characterised by a co-ordination number of 8. The radius ratio for CsCl, Cs /Cl" = 0.93, allows 8 co-ordination, but is so near the ratio for 6 co-ordination that caesium chloride is dimorphous, changing, at 445°, from the caesium chloride to the rock salt structure. The crystalline halides are generally markedly ionic, though, as expected, lithium iodide is somewhat covalent, for iodide is the largest and most easily polarised simple anion and lithium, the smallest alkali metal cation, possesses the strongest polarising power. 

Silver iodide is the only silver halide with an adamantine structure, the two elements having covalent radii in the ratio 1.34 1.33. Both AgCl (1.34 0.99) and AgBr (1.34 1.14) have a rock-salt structure and an ionic character. Gold (I) fluoride is unknown, and the chloride, bromide, iodide decrease in stability in that order, the formation of Aul being endothermic (d H, 5.52 kcal). With the exception of Aul, all are converted by water to the trihalide and metal, the chloride the most readily, possibly because the most soluble. 

Table 1 Summary of experimental information for Li , Cu" " and Ag" " impurities in atkab halide lattices. All lattices present rock salt structure except CsQ and CsBr that have CsCl-type structure.

One of the most widely known and used set of ionic radii are those estimated by Pauling [2] on the basis of interionic distances in ionic crystals. He noted that repulsive effects between ions of the same charge depend on the relative size of the cation and anion in the crystal, and also took into consideration the coordination number of the ion with oppositely charged neighbors in the crystal lattice. The results obtained for the alkali metal and halide ions for the case that the coordination number is six (rock salt structure) are summarized in table 3.1. 

MnS, MnSe and MnTe have the rock-salt structure. They are all strongly antiferromagnetic, as are also the anhydrous halides. The superexchange mechanism (page 602) is believed responsible for their antiferromagnetism. 

The simplest adsorption systems are those in which an inert molecule with a closed-shell electronic configuration is adsorbed on a regular neutral surface of an insulator or a large band gap semiconductor. The prototype for such a situation is the adsorption of CO on the (100) surface plane of aUcah halides or simple cubic metal oxides with rock-salt structure, such as MgO. Indeed, the adsorption of CO on the regular MgO(lOO) surface can be considered as the prototype for physisorption systems. It has been studied extensively both experimentally and theoretically the theoretical treatments comprise cluster approaches as well as periodic calculations, performed both by means of DFT and wave function based methods [70-84]. 

In fact, only CsCi, CsBr, and Csl, under normal conditions, possess the bcc structure. Cesium chloride crystallizes with the rock-salt structure (cfc) at temperatures above 445°C. This indicates that the bcc and cfc structures have similar energies. One should note in Table 14 the too-weak values obtained for LiC , LiBr, and Lil, which would crystallize in the zinc-blende or wurtzite structures. Finally, let us note that the distances between nearest neighbors in the arrangements bcc and cfc, observed in halides possessing both structures, are practically the same AgF, in spite of the value p 0.9, crystallizes in the cfc structure. 

All the alkali metal halides except CsCl, CsBr and Csl form cubic crystals with rock-salt structure, where each metal cation is surrounded by six halide anions, and each anion surrounded by six cations at the comers of an octahedron. See Fig. 5.3. Consultation of a textbook on elementary inorganic or physical chemistry should make it clear that the energy of one mole of the crystalline material relative to the separated, neutral atoms is given by... 

Fig. 5.4. M-X bond distances (Re) in 17 crystalline alkali metal halides with rock-salt structure as a function of the corresponding bond distance in the gaseous, monomeric MX molecules.

Table 5.2. Coordination numbers, C, Madelung constants, M, and calculated bond distance ratios, R R, for gaseous, monomeric alkali metal halides MX, gaseous square dimers M2X2, for cubic tetramers M4X4 and for MX crystals with rock-salt structures.

Color centers in alkali halide crystals are based on a halide ion vacancy in the crystal lattice of rock-salt structure (Fig. 5.76). If a single electron is trapped at such a vacancy, its energy levels result in new absorption lines in the visible spectrum, broadened to bands by the interaction with phonons. Since these visible absorption bands, which are caused by the trapped electrons and which are absent in the spectrum of the ideal crystal lattice, make the crystal appear colored, these imperfections in the lattice are called F-centers (from the German word Farbe for color) [5.138]. These F-centers have very small oscillator strengths for electronic transitions, therefore they are not suited as active laser materials. 

By 1920 the unit cell dimensions of 15 of the 17 alkali metal halides with the face-centered rock salt structures had been determined. The first attempt to utilize such data to estimate the size of individual ions appears to have been made by Alfred Lande [35]. He assumed that the alkali metal cations and halide anions could be regarded as spherical and concluded that the unit cell dimension a could be written... 

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