Crystal structures ionic

Finally, we should note that the lines that are often drawn in illustrations of three-dimensional ionic crystal structures to better show the relative arrangement of the ions do not represent shared pairs of electrons, that is, they are not bond lines. 

We know from quantum mechanics that atoms and ions do not have precisely defined radii. However, from the foregoing discussion of ionic crystal structures we have seen that ions pack together in an extremely regular fashion in crystals, and their atomic positions, and thus their interatomic distances, can be measured very accurately. It is a very useful concept, therefore, particularly for those structures based on close-packing, to think of ions as hard spheres, each with a particular radius. 

It is also possible to determine accurate electron density maps for the ionic crystal structures using X-ray crystallography. Such a map is shown for NaCl and LiF in Figure 1.45. The electron density contours fall to a minimum—although not to zero—in between the nuclei and it is suggested that this minimum position should be taken as the radius position for each ion. These experimentally determined ionic radii are often called crystal radii the values are somewhat different from the older sets and tend to make the anions smaller and the cations bigger than previously. The most comprehensive set of radii has been compiled by... 

This opening chapter has introduced many of the principles and ideas that lie behind a discussion of the crystalline solid state. We have discussed in detail the structure of a number of important ionic crystal structures and shown how they can be linked to a simple view of ions as hard spheres that pack together as closely as possible, but can also be viewed as the linking of octahedra or tetrahedra in various ways. Taking these ideas further, we have investigated the size of these ions in terms of their radii, and... 

ISODESMIC STRUCTURE. An ionic crystal structure in which there are no distinct groups formed within the structure, i.e.. where no bond is stronger than all the others. 

In the next section the ideas behind several methods, including the GA and a simulated annealing (SA) approach [19,20], then their implementation used to generate ionic crystal structures are reviewed. This will contain an introduction to the types of move class operators and the various types of cost functions used to modify the current trial structure(s) and to assess the quality of the trial structures, respectively. In the third section recent applications of the GA and SA approaches to closest-packed ionic systems and then to open-framework crystal structures are reviewed. 

As seen in Chapters 4 and 5, aqueous cations and anions are formed by the dissolution of metal oxides and acid phosphates. Electrostatic (Coulomb) force attracts the oppositely charged ions to each other and stacks them in periodic configurations. That results in an ionic crystal structure. Thus, the ionic bond is one of the main mechanisms that is responsible for forming the acid-base reaction products. 

Crystalline (monomeric) H3N. BH3 is a disordered crystal, and X-ray studies give B—N approximately 1-60 The other product, [H2B(NH3)2]C1 has a typical ionic crystal structure, consisting of layers of [H2B(NH3)2]ions (b) interleaved with cr ions. 

It is not in general possible for neutral atoms or molecules to be bound in ionic crystal structures, but the molecule of water, and to a lesser extent that of ammonia, are exceptions to this general rule. The reason for this anomalous behaviour lies primarily in the electrical polarity and small size of these molecules. The particular relevance of these two factors will appear as our discussion proceeds. 

Important Ionic Crystal Structures. Fig. 2-3 shows six of the most important structures found among essentially ionic substances. In an ionic structure each ion is surrounded by a certain number of ions of the opposite sign this number is called the coordination number of the ion. In the first three structures shown, namely, the NaCl, CsCl and CaF2 types, the cations have the coordination numbers 6, 8 and 8, respectively. 

A study of a number of other compounds which are generally considered to be among the most completely ionic shows that cubic coordination is not confined to the alkali halides. It occurs, for example, in the fluorite structure in which many ionic MXa compounds crystallize. On the other hand the isolated [Tab s]3 group has the square antiprism arrangement (64) predicted by the simple theory. This underlines a point of great importance to the understanding of ionic crystal structures, namely, that the requirement that a structure can be extended indefinitely in space imposes severe restrictions on the types of coordination which are possible. Cubic coordination can be extended indefinitely, but it is not possible to form an extended... 

An ideal crystal is constructed by the infinite regular repetition in space of identical structural units. In the simplest crystals formed by monatomic elements, the basic structural unit consists of a single atom. For the organic molecules of pharmaceutical interest, the structural unit will contain one or more molecules. One can describe the structure of all crystals in terms of a single periodic lattice, which represents the translational repetition of the fundamental structural unit. For elemental or ionic crystal structures, each point in the lattice may be a single atom. For organic molecules, a group of atoms is often attached to a lattice point or situated in an elementary parallelepiped. 

Nikolic K, Mumgesan M, Forshaw M et al (2007) Self-assembly of nanoparticles on the surface of ionic crystals structural properties. Surf Sci 601(13) 2730-2734... 

I) saltlike halides according to the formulation (R +)(X )2 with typically ionic crystal structures and... 

The structure of (CH )jNCl(SeOClj) illusuates that even larger neutral molecules than water and ammonia can be incorporated into an ionic crystal structure. Furthermore the arrangement around the anion (Cl") can be compared with that of cations in those hydrates for which the oxygen coordination number of the cations is greater than the available number of water molecules. 

The ionic crystals structure results from a pure geometrical settlement of ions with radii that are more or less different made in a way to obtain a structure that is as stable as possible from an energetic point of view. This condition makes that, for a certain t) e of lattice should correspond a very large number of combinations, if the ratio of the radii varies in small limits, and if the chaiges are identical. 

CTcloDentadlenvl-alkali Metal Compounds. SCF calculations on CpLl(NHj),j (n l-3) indicated the complexes to be largely ionic. Crystal structures have been determined for (1) [CpLi(12-crown-4)], a sandwich-like structure with Li co-ordinated on one side by 4 0 and on the other by Cp, (li) [1,2.4-(Me)Si)jC,Hi][Li(12-crown-4)j], [Cp ][LlDi] an ion-pair structure with a free [Cp ] anion, (iii) [Me CsK.3py] a 3-legged stool type structure and (iv) [MesCjK.2py] a linear zig-zag chain polymer. The nmr spectra of MesCjM in pyridine have also been reported. A shown by nau , (14) (R Li) exists in THF solution as an equilibrium mixture of an exo-lithio-monomer, an... 

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