Crystal structure cadmium iodide

Many inorganic solids crystallize in layer-lattice structures cadmium iodide is frequently cited as the type example. The cadmium ions are arranged hexagonally in sheets, each with a sheet of hexagonally arranged iodide ions above and below. The separation of adjacent sheets of... 

In other crystals an octahedral metal atom is attached to six non-metal atoms, each of which forms one, two, or three, rather than four, bonds with other atoms. The interatomic distance in such a crystal should be equal to the sum of the octahedral radius of the metal atom and the normal-valence radius (Table VI) of the non-metal atom. This is found to be true for many crystals with the potassium chlorostannate (H 61) and cadmium iodide (C 6) structures (Table XIB). Data are included in Table XIC for crystals in which a tetrahedral atom is bonded to a non-metal atom with two or three covalent bonds. The values of dcalc are obtained by adding the tetrahedral radius for the former to the normal-valence radius for the latter atom. 

FIGURE 1.40 (a) The crystal structure of cadmium iodide, CdU (b) the structure of the layers in CdU and CdCl2 the halogen atoms lie in planes above and below that of the metal... 

Consider a crystalline compound which no longer satisfies the requirements of a coordinated structure, but in which now each ion is symmetrically surrounded by a number of ions of opposite charge. Such a crystal is shown in Figure 24, and is actually that of cadmium iodide. In this crystal every cadmium ion is still surrounded in a... 

The octahedral radii of the table are applicable to complex ions such as [PtCle]—. The radius sum Pt(IV)—Cl is 2.30 A, and the several reported experimental values for salts of chloroplatinic acid range from 2.26 A to 2.35 A. The radii can also be applied to the sulfides, selen-ides, and tellurides of quadrivalent palladium and platinum (PdS2, etc.), which crystallize with the cadmium iodide structure, consisting of layers of MX octahedra so packed together that each X is common to three octahedral complexes. The average deviation between radius sums and reported distances for these substances is about 0.02 A. 

From the observed values of interatomic distances in complex ions such as [SnCh]—, [PbBr0], and [SeBr ]— and from crystals such as TiS2 with the cadmium iodide structure the octahedral radii given in Table 7-17 have been obtained. These correspond not to cPsp bonds, involving d orbitals of the shell within the valence shell, but to sp d2 orbitals, use being made of the unstable d orbitals of the valence shell itself. 

This brings us to a class of compounds too often overlooked in the discussion of simple ionic compounds the transition metal halides. In general, these compounds (except fluorides) crystallize in structures that are hard to reconcile with the structures of simple ionic compounds seen previously (Figs. 4.1-4.3). For example, consider the cadmium iodide structure (Fig. 7.8). It is true that the cadmium atoms occupy octahedral holes in a hexagonal closest packed structure of iodine atoms, but in a definite layered structure that can be described accurately only in terms of covalent bonding and infinite layer molecules. 

It is convenient here to consider the compound disilver monofluoride (or silver subfluoride). This was reported to have an anti-cadmium iodide structure in early work (5), and this has been confirmed more recently (6). From X-ray single-crystal results the unit cell is hexagonal, a = 2.996, c = 5.691 A, space-group C3m, and the Ag—F separation 2.451 A, almost identical with that in silver monofluoride (2.468 A). The structure consists of layers, with a plane of fluorine atoms sandwiched between two planes of silver atoms, giving the fluorine atom a coordination of six silver atoms, the same as in silver monofluoride. The silver atoms have separations of 2.996 and 2.814 A, compared with 2.889 A in the metal itself, and in line with the metallic conductivity of the compound. 

The hep structure consists of a stacking of atomic layers in the sequence ABABAB- . The octahedral holes are located between adjacent layers, as shown in Fig. 10.2.3(a). In the crystal structure of nickel asenide, the As atoms constitute a hep lattice, and the Ni atoms occupy all the octahedral holes. In contrast, cadmium iodide, Cdl2, may be described as a hep of 1 anions, in which only half the octahedral holes are occupied by Cd2+ ions. The manner of occupancy of the octahedral interstices is such that entire layers of octahedral interstices are filled, and these alternate with layers of empty... 

Sulphur is less electronegative than oxygen, and in consequence no sulphide of composition AS2 crystallizes with any of the typically ionic structures commonly found among the oxides A02. A number of sulphides have the cadmium iodide layer structure, but many others, particularly those of the transition metals, have structures unrepresented among the compounds so far considered. A few of these are of sufficiently common occurrence to warrant discussion. 

Ta2C has two crystal structures. The low temperature form shows an ordering of the carbon atoms such that the hexagonal metal layers are separated by alternately completely filled and completely empty carbon layers. This is the cadmium iodide antitype structure found by A. L. Bowman et al. (1965). However, the difference between the x-ray pattern of this phase and an L3 structure is very slight. Near 2000° (Rudy and Harmon, 1965) the carbon lattice presumably disorders to give the L 3 structure. 

The halides of cadmium demonstrate the effect on structure of the easier polarisation of an anion by a smaller cation. Cdp2 has the cubic fluorite lattice but the chloride, bromide and iodide form hexagonal crystals based on layer lattices (p. 150). The distances between the layers increase from the chloride to the iodide, with a corresponding reduction in lattice energies. 

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