Anion close packing

Fig. 6.4 Layered structure of LixTiSa, showing the lithium ions between the TiSa sheets. This is an anion close-packed lattice in which alternate layers between the anion sheets are occupied by a redox-active titanium atom. Lithium inserts itself into the empty remaining layers. (Adapted from [68])...

Fig. 2.5 Structure of lepidocrocite. a) Cubic close packed anion arrangement and distribution of cations over the octahedral interstices. Projection on (001). Octahedral arrangement and unit cell outlined, b) Projection of anion close packing on (010). Octahedral arrangement and unit cell outlined, c) Projection of anion close packing on (001). Dashed circles represent Fe in the next lo A/er layer, d) Arrangement of oc-...

Occurrence of layer types among structures based on anion close packing... 

In Table 6, we give a survey on the structures of halides, chalcogenides, pnictides and tetralides that are based on anion close-packing. Those which belong to a layer type are set in bold type. 

Fig. 104. (110) sections through the structures of the ZnIn2S4 polytypes I, Ila, Ilb and Ilia. The structures are idealized as anion close packings. Triangles and squares represent tetrahedral and octahedral cation sites, respectively. ZnIn2S4 Ilia is the inverse MgAl2S4 structure whose cell is... 

Above 200°C a-WCU transforms into jS-WC with the hexagonal UC structure. The high-temperature modification is based on the same hexagonal anion close-packing but the cations are now distributed between all layers. Thus the jS-WCle structure is in fact a NiAs-type derivative. 

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

Carbonyl hydrides and carbonylate anions are obtained by reducing neutral carbonyls, as mentioned above, and in addition to mononuclear metal anions, anionic species of very high nuclearity have been obtained, often by thermolysis. These are especially numerous for Rh and in certain Rh, Rh and Rhi5 anions have structures conveniently visualized either as polyhedra encapsulating further metal atoms, or alternatively as arrays of metal atoms forming portions of hexagonal close packed or body... 

The integrated charge would correspond to 0.7 0.1 Cu monolayers. Thus, either a less closely packed Cu layer or an anion co-adsorption that can both lead to a Moire superstructure are probed in the solution investigated [Al2Cl7] is the predominant anion. At h-200 mV vs. Cu/CW the superstructure disappears and a completely closed Cu monolayer is observed, with a charge corresponding to 1.0 0.1 Cu monolayers. 

In actual oxidation, the cubic anion lattice becomes extended by the addition of new layers of close-packed 0 ions into which Fe atoms migrate to give rise to the appropriate stable structures. 

A freshly prepared flame-annealed Au(100) surface has been found to be reconstmcted188,487,534,538 and the surface atoms exhibit a hexagonal close-packed structure to yield the (hex)-stmcture. One-directional long-range corrugation of 1.45 nm periodicity and 0.05 nm height has been found on the Au( 100) surface.188,488 When the reconstruction is lifted due to specific adsorption of SO - anions at more positive , the surface changes to a (1 x 1) structure.538... 

The rock-salt structure is a common ionic structure that takes its name from the mineral form of sodium chloride. In it, the Cl- ions lie at the corners and in the centers of the faces of a cube, forming a face-centered cube (Fig. 5.39). This arrangement is like an expanded ccp arrangement the expansion keeps the anions out of contact with one another, thereby reducing their repulsion, and opens up holes that are big enough to accommodate the Na+ ions. These ions fit into the octahedral holes between the Cl ions. There is one octahedral hole for each anion in the close-packed array, and so all the octahedral holes are occupied. If we look carefully at the structure, we can see that each cation is surrounded by six anions and each anion is surrounded by six cations. The pattern repeats over and over, with each ion surrounded by six other ions of the opposite charge (Fig. 5.40). A crystal of sodium chloride is a three-dimensional array of a vast number of these little cubes. 

When the radius ratio of an ionic compound is less than about 0.4, corresponding to cations that are significantly smaller than the anion, the small tetrahedral holes may be occupied. An example is the zinc-blende structure (which is also called the sphalerite structure), named after a form of the mineral ZnS (Fig. 5.43). This structure is based on an expanded cubic close-packed lattice of the big S2 anions, with the small Zn2+ cations occupying half the tetrahedral holes. Each Zn2+ ion is surrounded by four S2 ions, and each S2" ion is surrounded by four Zn2+ ions so the zinc-blende structure has (4,4)-coordination. 

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 ... 

Many ionic compounds are considered to pack in such as way that the anions form a close-packed lattice in which the metal cations fill holes or interstitial sites left between the anions. These lattices, however, may not necessarily he as tightly packed as the label close-packed implies. The radius of an F ion is approximately 133 pm. The edge distances of the cubic unit cells of LiF, NaF, KF, RbF, and CsF, all of which... 

Ziegler-Natta catalyst A stereospecific catalyst for polymerization reactions, consisting of titanium tetrachloride and triethylaluminum. zinc-blende structure A crystal structure in which the cations occupy half the tetrahedral holes in a nearly close packed cubic lattice of anions also known as sphalerite structure. 

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