Cadmium lattice structure

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... 

However, It Is well to note that Na is not an activator in the strictest sense but is a member of the lattice structure, placed there to avoid formation of a lattice vacancy. If a -vacancy is formed, it is likely that the resulting "phosphor" may be non-luminescent or will exhibit a low degree of luminescent efficiency. Although you will find that many phosphors like ZnS Ag, Al K are listed as requiring the K activator, in reality this cation is present for charge compensation in the lattice. This fact has not been recognized by many prior workers in the field. Furthermore, some zinc (cadmium) sulfides have several listed "activators", part of which are undoubtedly there for charge compensation. Elxamples Include ... 

Kitazawa. T. Nishikiori. S. Kuroda. R. Iwamoto. T. Clathrate compounds of cadmium cyanide and related hosts with cristobalite-like lattice structures. J. Chem. Soc., Dalton Trans. 1994. 1029-1036. 

Building on the proposed mechanism hy Hauffe, ° for metal oxide-catalysed dehydration of alcohols to form olefins, ethers and water which heavily relies on the assumption that the catalyst surface acts as semiconductor, Hasssan et al. purposively attempted to obtain a further insight into the mechanism of alcohol dehydration on pure cadmium oxide. On the basis of the experimental data involving kinetics of the dehydration reaction and the effect of pretreatment of the catalysts along with studies on lattice structure and specific surface areas, a mechanism for ethanol dehydration was put forward (Scheme 17.14). The proposed mechanism entirely depends... 

Solid cadmium(II) iodide Cdlj has a layer lattice —a structure intermediate between one containing Cd " and P ions and one containing Cdl2 molecules—and this on vaporisation gives linear, covalent I—Cd—I molecules. In solution, iodo-complexes exist, for example... 

On Cu(lll) different structures were proposed. The bulk deposited cadmium forms a close-packed hexagonal lattice in perchlorate solutions, growing according to modified Stranski-Krastanov mechanism [292]. 

Electron microscopy has been performed using a sample synthesised at w = 10, [Cd2+]/[S2 ] = 2, and characterized by 430-nm absorption onset, which corresponds to a CdS diameter equal to 25 A. The microanalysis study shows the characteristic lines of sulfide and cadmium ions, indicating that the observed particles are CdS semiconductor crystallites. The electron diffractogram shows concentric circles, which are compared to a simulated diffractogram of bulk CdS. A good agreement between the two spectra is obtained, indicating the particles keep zinc-blend crystalline structure (fee) with a lattice constant equal to 5.83 A. 

Type B oxides have a metal excess which is incorporated into the lattice in interstitial positions. This is shown in (b)(1) as an interstitial atom, but it is more likely that the situation in (b)(ii) will hold, where the interstitial atom has ionised and the two electrons so released are now associated with two neighbouring ions, reducing them from to M. Cadmium oxide, CdO, has this type of structure. Oxygen is lost when zinc(II) oxide is heated, forming Zn +JD, oxide vacancies form and to compensate, Zn ions migrate to interstitial positions and are reduced to Zn ions or Zn atoms. Electron transfer can take place between the Zn and ZnTZn resulting in the yellow coloration seen when ZnO is heated. 

Figure 27. Crystal lattice of cadmium pigments (wurtzite structure) a) Sulfur (selenium) b) Cadmium (zinc, mercury)...

When hydrogen sulfide is passed into cadmium salt solutions, cadmium sulfide is formed as a yellow precipitate with a zinc blende structure (cubic, (i-form). The P-form can be converted into the a-form (e.g., by heating). a-Cadmium sulfide shows photoconductivity due to defects in the crystal lattice (usage in photovoltaic cells) [3.106]. The solubility in water at 25°C is 1.46 x 10 10 mol/L [3.107], Cadmium sulfide forms the basis for all cadmium pigments. 

The luminescent properties can be influenced by the nature of the activators and coactivators, their concentrations, the composition of the flux, and the firing conditions. In addition, specific substitution of zinc or sulfur in the host lattice by cadmium or selenium is possible, which also influences the luminescent properties. Zinc sulfide is dimorphic and crystallizes below 1020 °C in the cubic zinc-blende structure and above that temperature in the hexagonal wurtzite lattice. When the zinc is replaced by cadmium, the transition temperature is lowered so that the hexagonal modification predominates. Substitution of sulfur by selenium, on the other hand, stabilizes the zinc-blende lattice. 

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... 

The random solution of defects caused by lattice dissociation and nonstoichiometry has been discussed for CeCd 4 5. In addition to this random solution in an otherwise ordered lattice, these defects can themselves order to create very many new and complex ordered lattices which we have named microphases. Structures with very similar properties appear at cadmium concentrations both greater and less than that of CeCd4 5 if the CeCd4 5 structure were the common base of all the microphases, discontinuities would be expected at CeCd4>5 where the composition shifted from excess cadmium to deficient cadmium. CeCd4, however, is a reasonable common base. The microphases are the subject of another paper (6). 

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