Lattices ionic structures

A detailed discussion of individual halides is given under the chemistry of each particular element. This section deals with more general aspects of the halides as a class of compound and will consider, in turn, general preparative routes, structure and bonding. For reasons outlined on p. 805, fluorides tend to differ from the other halides either in their method of synthesis, their structure or their bond-type. For example, the fluoride ion is the smallest and least polarizable of all anions and fluorides frequently adopt 3D ionic structures typical of oxides. By contrast, chlorides, bromides and iodides are larger and more polarizable and frequently adopt mutually similar layer-lattices or chain structures (cf. sulfides). Numerous examples of this dichotomy can be found in other chapters and in several general references.Because of this it is convenient to discuss fluorides as a group first, and then the other halides. 

The packing in ionic crystals requires that ions of opposite charges alternate with one another to maximize attractions among ions. A second important feature of ionic crystals is that the cations and anions usually are of different sizes. Usually the cations are smaller than the anions. Consequently, ionic compounds adopt a variety of structures that depend on the charges and sizes of the ions. One way to discuss ionic structures is to identify a crystal lattice for one set of ions, and then describe how the other ions pack within the lattice of the first set. 

When spherical objects are stacked to produce a three-dimensional array (crystal lattice), the relative sizes of the spheres determine what types of arrangements are possible. It is the interaction of the cations and anions by electrostatic forces that leads to stability of any ionic structure. Therefore, it is essential that each cation be surrounded by several anions and each anion be surrounded by several cations. This local arrangement is largely determined by the relative sizes of the ions. The number of ions of opposite charge surrounding a given ion in a crystal is called the coordination number. This is actually not a very good term because the bonds are not coordinate bonds (see Chapter 16). For a specific cation, there will be a limit to the number of anions that can surround the cation because... 

The van der Waals-London forces play a particularly important part in the formation of molecular lattices even when the molecules still have an ionic structure screening and polarization diminish the effect of electrostatic forces to a great extent, and the van der Waals forces are therefore preponderant. Examples of substances which form molecular lattices are those of the type XY4, such as CC14, CI4, SnCl4, etc. In these a small positive ion is surrounded by four large halogen ions in the form of a tetrahedron, with the result that the external electric field of the positive ion is very weak. Mutual attraction of molecules is often exclusively the result of the van der... 

Position of Metal Ions in Monolayer Lattice. The surface potential is an important parameter for studying the ionic structure of monolayers, including the position of metal ions in the monolayer lattice. The interaction of cations with anionic groups in a monolayer results in a formation of ionic dipoles which influence the surface potential. If the polarity of the ionic dipole is in the same direction as that of the rest of the molecule, the surface potential of the monolayer increases if the polarities are opposite, the surface potential decreases. It is known (21, 41, 46)... 

Like infrared spectrometry, Raman spectrometry is a method of determining modes of molecular motion, especially the vibrations, and their use in analysis is based on the specificity of these vibrations. The methods are predominantly applicable to die qualitative and quantitative analysis of covalently bonded molecules rather than to ionic structures. Nevertheless, they can give information about the lattice structure of ionic molecules in the crystalline state and about the internal covalent structure of complex ions and the ligand structure of coordination compounds both in the solid state and in solution. 

Figure 3.l5d shows only a tiny part of a small crystal of sodium chloride. Many millions of sodium ions and chloride ions would be arranged in this way in a crystal of sodium chloride to make up the giant ionic structure. Each sodium ion in the lattice is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions. 

Giant ionic structure A lattice held together by the electrostatic forces of attraction between ions. 

The reader is probably familiar with a simple picture of metallic bonding in which we imagine a lattice of cations M"+ studded in a sea of delocalised electrons, smeared out over the whole crystal. This model can rationalise such properties as malleability and ductility these require that layers of atoms can slide over one another without-undue repulsion. The sea of electrons acts like a lubricating fluid to shield the M"+ ions from each other. In contrast, distortion of an ionic structure will necessarily lead to increased repulsion between ions of like charge while deformation of a molecular crystal disrupts the Van der Waals forces that hold it together. It is also easy to visualise the electrical properties of metals in... 

There is no regularity in the atomic or ionic structure of a glass. Every time a glass solidifies, it can assume a different shape. When nonglass substances solidify, they form into a regular, predictable lattice. 

There are two general interpretations for the admittance characteristics of barrier anodic films (98). The first interpretation is based on film conduction mechanisms, either electronic or ionic, and the influence of solution ions on the oxide film lattice defect structure on conduction behavior. The second is based on the behavior of preexisting defects in anodic films and the effects of attacking or passivating solutions. 

The lattice energy based on the Born model of a crystal is still frequently used in simulations [14]. Applications include defect formation and migration in ionic solids [44,45],phase transitions [46,47] and, in particular, crystal structure prediction whether in a systematic way [38] or from a SA or GA approach [ 1,48]. For modelling closest-packed ionic structures with interatomic force fields, typically only the total lattice energy (per unit cell) created by the two body potential,... 

Another difference is that the 5/orbitals have a greater spatial extension relative to the Is and Ip orbitals than the 4/orbitals have relative to the 6s and 6p orbitals. The greater spatial extension of the 5/orbitals has been shown experimentally the esr spectrum of UF3 in a CaF2 lattice shows structure attributable to the interaction of fluorine nuclei with the electron spin of the U3+ ion. This implies a small overlap of 5/ orbitals with fluorine and constitutes an / covalent contribution to the ionic bonding. With the neodymium ion a similar effect is not observed. Because they occupy inner orbitals, the 4/ electrons in the lanthanides are not accessible for... 

CoYiilcncy in hondiny leads to deviations from ealculated lattice enthalpies based on ionic structures. 

In KgPtCl there exists resonance between the covalent and ionic structures. The KgPtCl crystal consists of K+ ions and octahedral PtCl -ions arranged in a similar manner to the Li+ and ions in an antifluorite lattice. Each potassium ion is surrounded by twelve chlorine atoms and the closeness of the packing will evidently be due to the attraction between the positive potassium ions and the partially negative atoms of chlorine. 

Most salts crystallize as ionic solids with ions occupying the unit cell. Sodium chloride (Figure 13-28) is an example. Many other salts crystallize in the sodium chloride (face-centered cubic) arrangement. Examples are the halides of Li+, K+, and Rb+, and M2+X2 oxides and sulfides such as MgO, CaO, CaS, and MnO. Two other common ionic structures are those of cesium chloride, CsCl (simple cubic lattice), and zincblende, ZnS (face-centered cubic lattice), shown in Figure 13-29. Salts that are isomorphous with the CsCl structure include CsBr, Csl, NH4CI, TlCl, TlBr, and TIL The sulfides of Be2+, Cd2+, and Hg2+, together with CuBr, Cul, Agl, and ZnO, are isomorphous with the zincblende structure (Figure 13-29c). 

Fig. 3.08. The electrostatic component of the lattice energy of the caesium chloride, sodium chloride and zincblende ionic structures as a function of the radius ratio r+/r (r assumed constant). The energy values are negative on an arbitrary scale, the zero of energy being above the top of the figure.

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