Lattices ionic

Dynamic models for ionic lattices recognize explicitly the force constants between ions and their polarization. In shell models, the ions are represented as a shell and a core, coupled by a spring (see Refs. 57-59), and parameters are evaluated by matching bulk elastic and dielectric properties. Application of these models to the surface region has allowed calculation of surface vibrational modes [60] and LEED patterns [61-63] (see Section VIII-2). 

These solid ionic hydrides (having an ionic lattice and containing the hydride ion H ) react with water, for example... 

The white solid oxides MjO and M 0 are formed by direct union of the elements. The oxides MjO and the oxides M"0 of calcium down to radium have ionic lattices and are all highly basic they react exothermically with water to give the hydroxides, with acids to give salts, and with carbon dioxide to give carbonates. For example... 

However, phosphorus pentachloride in the solid state has an ionic lattice built up of (PC ) and (PClg)" ions and these ions are believed to exist in certain solvents. Thus under these conditions the maximum covalency is reached with chlorine. In phosphorus pentabromide, PBrj, the solid has the structure [PBr4] [Br] . 

ChloricfVII) acid fumes in moist air and is very soluble in water, dissolving with the evolution of much heat. Several hydrates are known the hydrate HCIO4. H2O is a solid at room temperature and has an ionic lattice [HjO ] [CIO4]. 

These are halides formed by highly electropositive elements (for example those of Groups I and II, except for beryllium and lithium). They have ionic lattices, are non-volatile solids, and conduct when molten they are usually soluble in polar solvents in which they produce conducting solutions, indicating the presence of ions. 

Much work has been done on the structure of the metal alkoxides (49). The simple alkaU alkoxides have an ionic lattice and a layer stmcture, but alkaline earth alkoxides show more covalent character. The aluminum alkoxides have been thoroughly studied and there is no doubt as to their covalent nature the lower alkoxides are associated, even in solution and in the vapor phase. The degree of association depends on the bulkiness of the alkoxy group and can range from 2 to 4, eg, the freshly distilled isopropylate is trimeric (4) ... 

The most symmetrical structure possible is the cube Oh but, except in extended ionic lattices such as those of CsCl and CaF2, it appears that inter-ligand repulsions are nearly always (but see p. 1275) reduced by distorting the cube, the two most important resultant structures being the square antiprism D4h and the dodecahedron Did (Fig. 19.10). 

We have considered the weak van der Waals forces that cause the condensation of covalent molecules. The formation of an ionic lattice results from the stronger interactions among molecules with highly ionic bonds. But most molecules fall between these two extremes. Most molecules are held together by bonds that are largely covalent, but with enough charge separation to affect the properties of the molecules. These are the molecules we have, called polar molecules. 

Only AuF of the gold(I) halides is unknown in the solid state its stability can be examined by means of a Bom-Haber cycle, assuming that it would have an ionic lattice like AgF. (AuF has been generated in the gas phase from Au+ and CH3COF [22].)... 

Fig. 10.3 Ideas concerning crystals /e/ the continuous model of Hairy with cubic particles, right ihe nowadays model of ionic lattice...

Based on the concept of mixed-framework lattices, we have reported a novel class of hybrid solids that were discovered via salt-inclusion synthesis [4—7]. These new compounds exhibit composite frameworks of covalent and ionic lattices made of transition-metal oxides and alkali and alkaline-earth metal halides, respectively [4]. It has been demonstrated that the covalent frameworks can be tailored by changing the size and concentration of the incorporated salt. The interaction at the interface of these two chemically dissimilar lattices varies depending upon the relative strength of covalent vs. ionic interaction of the corresponding components. In some cases, the weak interaction facilitates an easy... 

We have touched briefly on three simple ionic lattices, but there are many others. Moreover, the stmctures of many crystalline network solids can also be described by the methods we have introduced here for NaCl, CsCl, and CaF2. 

Componnds forming ionic crystals (e.g., NaCl). In them the ions exist even prior to dissolntion, but are held in lattice sites, owing to electrostatic interaction. In ionic lattices covalent bonds between the ions are practically nonexistent. These lattices disintegrate during dissolntion and the ions become mobile (free). Snch substances are called ionophors. 

Point defects in solids make it possible for ions to move through the structure. Ionic conductivity represents ion transport under the influence of an external electric field. The movement of ions through a lattice can be explained by two possible mechanisms. Figure 25.3 shows their schematic representation. The first, called the vacancy mechanism, represents an ion that hops or jumps from its normal position on the lattice to a neighboring equivalent but vacant site or the movement of a vacancy in the opposite direction. The second one is an interstitial mechanism where an interstitial ion jumps or hops to an adjacent equivalent site. These simple pictures of movement in an ionic lattice, known as the hopping model, ignore more complicated cooperative motions. 

Fumi F. G. and Tosi M. P. (1957). Naor relations between Madelung constants for cubic ionic lattices. Phil Mag, 2 284-285. 

In Chapters 2 and 3 we have described basic structural properties of the components of an interphase. In Chapter 2 we have shown that water molecules form clusters and that ions in a water solution are hydrated. Each ion in an ionic solution is surrounded predominantly by ions of opposite charge. In Chapter 3 we have shown that a metal is composed of positive ions distributed on crystal lattice points and surrounded by a free-electron gas which extends outside the ionic lattice to form a surface dipole layer. 

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