Cubic close-packing stability

MnS and MnSe are the only transition-element compounds which have a zinc blende modification. The ZnS structure is the cubic version of the ZnO structure, i. e. the cations occupy half the tetrahedral holes in a cubic close-packed anion sublattice. As in the rocksalt structure the anion and the cation sublattices are identical to one another, i.e. the NaCl and ZnS structures are their own antitype. Like in the case of ZnS itself one should expect several polytypes to occur for MnS and MnSe. MnTe can be stabilized in the zinc blende structure by adding B3-type tellurides. Cubic mixed crystals Zni Mn Te were synthesized up to x = 0.86 171), Cdi-zMnzTe up to x — 0.75 172) and Hgi- Mn Te up to x = 0.8 172). 

Nanocrystals with a specific number of atoms (nuclearity) are bequeathed special stability [7]. For nanocrystals of cubic close-packed metals, the numbers 13, 55,147, 309 and 561 stand for magic nuclearities corresponding to the closure of 1, 2, 3, 4 and 5 shells respectively. A schematic illustration of magic nuclearity nanocrystals is shown in Fig. 8.1. In Fig. 8.2, we show scanning tunneling and transmission electron micrographs of polymer-protected Pd561 nanocrystals. 

For the beautiful tetracapped octahedral Os cluster, [OsioC(CO)24]2- with an interstitial C atom in the octahedral core, shown in Figure 3.10, the predicted eve count is 14(6) + 2 + 4(12) = 134, which agrees with that of the observed stoichiometry. It s a little bit harder to count the sep but give it a try. Each tetrahedral cap consists of an Os(CO)3 fragment and the other six fragments are Os(CO)2 so we have (4x2 + 6x0 + 4 + 2)/2 = 7 appropriate for an octahedron. If you look ahead in Chapter 6 (Exercise 6.1), you will find that this trigonal bipyramidal ten-atom core can be excised from a cubic close-packed metal lattice (ABC layers). [OsioC(CO)24]2- can be considered a nano-sized metal particle stabilized by the ligands in the same manner as Ni atoms are stabilized when removed from Ni metal by CO as Ni(CO)4 in the Mond process. 

When the a phase, i.e, the primary solid solution, has only a limited range of stability, other intermediate phases are formed. At particular concentrations of the second component a transformation from one crystal structure to another takes place. In a large number of binary systems, e.g. Gu-Au, Cu-Al, Cu-Sn, a transition from the cubic close packed structure of copper to a body centred cubic structure ()3 phase) occurs at a particular concentration. The phase is stable over a particular range of concentration and at higher concentrations is generally converted to the y-phase which has a complex structure, followed by the e and >) phases which are... 

In addition, the smaller size of Al + and its lack of crystal field stabilization energy permits the stability of tetrahedral coordination of A13+ to approach that of octahedral. In Cr +, tetrahedral coordination is much less stable. Therefore, a series of aluminas are known based upon a cubic close-packed lattice of oxide ions in which some A13+ occupies tetrahedral holes (87, 88). All catalytically important aluminas... 

Some ternary and mixed-valency oxides have the spinel structure where metal ions occupy a proportion of tetrahedral and octahedral holes in a cubic close-packed lattice (see Topic G5). Examples include M304 with M=Mn, Fe, Co. The distribution of M2+ and M3+ ions between the tetrahedral and octahedral sites shows the influence of ligand field stabilization energies (see Topic H2). In Fe304, Fe2+ (3d6) has an octahedral preference whereas Fe3+ (3d5) has none, and this... 

When or-Fe203 was used as the positive electrode in high-temperature lithium cells, the introduction of a small amount of lithium into the corundum-type structure caused the hexagonal-close-packed oxygen array to shear irreversibly to cubic-close packing which generated a defect /-Li FCjOj (spinel-type) structure. Further lithiation resulted in the formation of LiFe,Ojj thereafter, the reaction followed the same sequence as that shown in reactions (4), (5) and (6) [100]. The stability of the spinel structures at elevated temperatures, as well as the ability of the cubic close-packed oxygen array to accommodate lithium at the expense of... 

B. Stability of Cubic Close Packing for Heavy Rarc-Gas... 

There have been several attempts to explain the stability of the cubic close packing for rare-gas atoms over the hexagonal closepacking (excepting helium for which quantum effects are predominant). 

Interactions between second and farther neighbors were calculated in detail by Kihara and Koba. It was shown, however, that the stability of the cubic close-packed lattice does not follow from the additive potential... 

Thus we reach the following conclusion The stability of the cubic close-packed structttre of heavy rare-gas crystals is due to the repulsion between electrical octupoles induced in atoms in the hexagonal case. 

The structures of spinels, A B2 4, are determined not only by the radius of the ions involved but also by the crystal field stabilization energies of the cations that occupy octahedral or tetrahedral holes in the cubic close-packed lattice of oxide ions. These structures offer an opportunity to combine a knowledge of crystal field theory obtained in earlier chapters with the knowledge of solid-state structures covered in this chapter. 

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