Ionic crystals tables

Structure of Other Simple Ionic Crystals Table 13-15.—Interatomic Distances M—X for Liganct 12... 

A similar calculation can be carried out for ionic crystals (Table 8.2). In this case, the Coulomb interaction is taken into account, in addition to the van der Waals attraction and the Pauli repulsion. Although the van der Waals attraction contributes htde to the three-dimensional lattice energy, its contribution to the surface energy is significant and typically 20-40% [838]. The calculated surface energy is sensitive to the particular choice of the interatomic potential. 

The uncertainties in choice of potential function and in how to approximate the surface distortion contribution combine to make the calculated surface energies of ionic crystals rather uncertain. Some results are given in Table VII-2, but comparison between the various references cited will yield major discrepancies. Experimental verification is difficult (see Section VII-5). Qualitatively, one expects the surface energy of a solid to be distinctly higher than the surface tension of the liquid and, for example, the value of 212 ergs/cm for (100)... 

From these various examples, it is clear that the adsorption energy for a given kind of site can vary quite markedly from one crystal face of the adsorbent to another. For argon on solid xenon (Table 1.1), for example, the most favourable site has a o value of —1251 x 10" J on the (100) face but only -1072 on the (111) face. Such differences are in no way surprising, and they have been found also with ionic crystals. 

Lanthanides Elements 57 (La) through 70 (Yb) in the periodic table, 146 Lanthanum, 147 Laser fusion, 528 Lattices in ionic crystals, 249 Lavoisier, Antoine, 14 Law of conservation of energy A natural law stating that energy can neither be created nor destroyed it can only be converted from one form to another, 214... 

Of the three principal classes of crystals, ionic crystals, crystals containing electron-pair bonds (covalent crystals), and metallic crystals, we feel that a good understanding of the first class has resulted from the work done in the last few years. Interionic distances can be reliably predicted with the aid of the tables of ionic radii obtained by Goldschmidt1) by the analysis of the empirical data and by Pauling2) by a treatment based on modem theories of atomic structure. The stability,... 

Table 7.13 Hardness (Mohs Scale) of Some Ionic Crystals.

Table 1. Quantitative data of structural k - model for the ionic crystals LiF, NaF, and MgO ...

The object of this paper is to discuss some of these problems. We start with the evidence for the new system of ionic radii. X-rays are diffracted by electrons in principle therefore X-ray diffraction should always locate the few outer electrons involved in bonding, but in fact this requires sophisticated treatment of meticulous measurements on crystals of high symmetry [6—9). But it has long been clear that some ionic crystals show round each ion an electron density which is approximately spherical but falls away to a very low background the following Table shows a typical example. 

It is evident that the electrostatic interactions constitute a major component of the lattice energy of ionic crystals. According the treatment for NaF described above, the ratio of absolute values of the electrostatic and repulsive forces to the lattice energy is l l/n, where n is the Born coefficient. With n ff = 7.445 (Table... 

A for the more important ionic crystals are given in Table 13-1. The magnitudes of the A values are seen to be reasonable on comparison with those for finite molecules A for an isolated molecule Na+Cl is 1, the Coulomb energy being — 1 -eV-Ko, whereas for a odium chloride crystal with the same interionic distance the crystal energy is about 75 percent greater, the value of Aat being 1.74756. 

In the general theory of ionic crystals (such as table salt, NaCl), a key physical quantity is the cohesive energy Zsxtal of forming the solid crystal from its constituent ions. For sodium chloride, for example, this is the energy lowering in the reaction... 

An ionic compound typically contains a multitude of ions grouped together in a highly ordered three-dimensional array. In sodium chloride, for example, each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions (Figure 6.11). Overall there is one sodium ion for each chloride ion, but there are no identifiable sodium-chloride pairs. Such an orderly array of ions is known as an ionic crystal. On the atomic level, the crystalline structure of sodium chloride is cubic, which is why macroscopic crystals of table salt are also cubic. Smash a large cubic sodium chloride crystal with a hammer, and what do you get Smaller cubic sodium chloride crystals Similarly, the crystalline structures of other ionic compounds, such as calcium fluoride and aluminum oxide, are a consequence of how the ions pack together. 

From the van der Waals radii of Table 2-1 and the ionic crystal radius of Ca2+ of 0.10 nm, we can estimate an approximate distance between the centers of positive and negative charge of 0.25 nm in both cases. It is of interest to apply Coulomb s law to compute the force F between two charged particles which are almost in contact. Let us choose a distance of 0.40 nm (4.0 A) and apply Eq. 2-7. 

TABLE 6 IONIC CRYSTAL RADII (IN ANGSTROM UNITS)-... 

Trends in Lattice Energy. We have seen that the lattice energy of ionic crystals is affected to some extent by the coordination numbers of the ions (Table 3.4) and by repulsion between ions in contact with each other (Eq. 3.14). These factors are, however, of minor importance when compared to the effect of ionic charge and ionic size. 

In order to be able to describe the ideal crystal structure , it is important to bear in mind that there are two tetrahedral cavities and one octahedral one present for every sphere in a close-packed structure. With the help of Table 4.1 and the rules for octahedral and tetrahedral coordination the description of the following crystal structures are now easily understood when we bear in mind that in an ionic crystal lattice the larger negative ions form the close-packed structure and that the octahedral and tetrahedral cavities are filled with positive ions. 

Table 8.3 Calculated surface tensions 7 and surface stresses T of ionic crystals for different surface orientations compared to experimental results [325,327]. All values are given in mN/m. (a) from cleavage experiments, (b) extrapolated from liquid.

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