Anion packing

One of the major factors in determining the structures of the substances that can be thought of as made up of cations and anions packed together is ionic size. It is obvious from the nature of wave functions that no ion has a precisely defined radius. However, with the insight afforded by electron... 

Water [579] is present in the structure of true crystalline hydrates [580] either as ligands co-ordinated with the cation (e.g. [Cu(OH2)4]2+ in CuS04 5 H20) or accommodated outside this co-ordination sphere within voids left in anion packing, further stabilized by hydrogen bonding (e.g. the remaining water molecule in CuS04 5 H20). 

Tetrahedral and octahedral interstitial holes are formed by the vacancies left when anions pack in a ccp array, (a) Which hole can accommodate the larger ions (b) What is the size ratio of the largest metal cation that can occupy an octahedral hole to the largest that can occupy a tetrahedral hole while maintaining the close-packed nature of the anion lattice (c) If half the tetrahedral holes are occupied, what will be the empirical formula of the compound MVAV, where M represents the cations and A the anions ... 

A sodium chloride crystal contains equal numbers of Na" cations and Cl anions packed together in an alternating cubic array. Figure 2-24 illustrates a portion of the sodium chloride array. Electrical forces hold the cations and anions in place. Each Na cation attracts all the nearby Cl anions. Likewise, each Cl anion attracts all its Na neighbors. Positive cations and negative anions group together in equal numbers to make the entire collection neutral. 

When three different kinds of spherical ions are present, their relative sizes are also an important factor that controls the stability of a structure. The PbFCl type is an example having anions packed with different densities according to their sizes. As shown in Fig. 7.5, the Cl- ions form a layer with a square pattern. On top of that there is a layer of F ions, also with a square pattern, but rotated through 45°. The F ions are situated above the edges of the squares of the Cl- layer (dotted line in Fig. 7.5). With this arrangement the F -F distances are smaller by a factor of 0.707 (= /2) than the CP-CP distances this matches the ionic radius ratio of rF-/rcl- = 0.73. An F layer contains twice as many ions as a CP layer. Every Pb2+ ion is located in an antiprism having as vertices four F and four... 

The strong affinity of ionic liquids for aromatics has been attributed to the formation of liquid clathrates (90-92). Liquid clathrates (93) are semi-ordered liquids containing complex salt hosts. They are formed by associative interactions between aromatic molecules and salt ions, which separate cation-anion-packing interactions to a sufficient degree that localized cage structures are formed. Although the aromatic compounds are highly soluble in the ionic liquid phase, the... 

Wustite, FeO, is the other iron oxide which contains only divalent Fe. It is usually non-stoichiometric (O-defident). The structure is similar to that of NaCl and is based on ccp anion packing. Wustite is black. It is an important intermediate in the reduction of iron ores. 

The natural and synthetic compounds differ in the arrangement of cations in both compounds the anion packing is disordered. Feroxyhyte is a disordered form of the synthetic material with the cations being distributed almost entirely randomly over the interstices. 

Decomposition of akaganeite starts at 150 °C and complete conversion to hematite is achieved at ca. 500 °C. This is not a topotactic transformation it involves a complete breakdown of the bcc anion packing of akaganeite followed by reconstruction of the hep anion array of hematite. Initially, the product is in the form of elongated, porous... 

Thus, the difference between these structures and the apatites is simply in the choice of tetrahedral interstices in the alloy which are used to accommodate the anions. This determines the type of cation-centred polyhedron for the minority cation. (We ignore the trivial variations in anions inserted into the [OOz] tunnels.) This suggests that the substitution of C03 for P04 in apatites may well be a less radical change than is usually assumed (when anion packing is accepted as being dominant). 

Anion Packing Coordination Number of M and 0 Sites by Cations Structure Name Examples... 

FIGURE 1.46 (a) Anions packed around a cation on a horizontal plane, (b) anion-anion contact on a horizontal... 

Fig. 2.105 Model structure with iw = 11 = 11 x 1 (see Fig. 2.102(c)).5 Anion packing distortion, shown by gaps between adjacent pentagonal bipyramids, is evident near the folding plane.

Fig. 2.106 Four types of relief from distortion of anion packings. In each (a) is shown the ideal structure. White circles are the oxygens on (010) to be eliminated. In (b) is the real structure after elimination of oxygens. Each of the structures (b) is denoted as DP(i), DP(ii), DP(iii), and DP(iv), respectively.

Some limiting cases are noteworthy. 1) The most frequent case is the oxidation of metals leading to semiconducting oxides with dense anion packing, that is, fel = 1 and D0 = 0. This gives... 

When the large heteropolymolybdate and heteropolytungstate anions pack together as units in the crystal, the interstices between the anions are very large compared either to water molecules or to most simple cations. In most compounds there is apparently no direct linkage between the individual heteropoly anions as shown in... 

TisSs (188) is based on the same anion packing (cchh)a but now cations are missing in the three- membered chain units along the c-axis ... 

The halides TIX3 (X = I, Br, Cl, I) provide an interesting series which couples both anion packing forces and the electronic stability of Tl against tF. TIF3 adopts the YF3... 

Ag(II) is in a highly polar electric field, since all four nearest boron atoms of each Ag(II) are either above or below it with respect to the chain direction. Nevertheless, the two intrachain Ag-F distances are remarkably close at 2.002(3) and 2.009(3) A. (The difference can be readily reconciled by the way in which the anion packing dictates that the four additional anion F ligands directed toward each Ag(II) are not in exactly the same plane as it.)... 

Figure 1.21 Cubic arrangement of eight anions packed around a central cation...

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