Chain layer

Some metahrich silicides have isolated Si atoms and these occur either in typical metallike structures or in more polar structures. With increasing Si content, there is an increasing tendency to catenate into i.solated Si2 or SU, or into chains, layers or 3D networks of Si atoms. Examples are in Table 9.3 and further structural details are in refs. 24, 26 and 27. 

Structural classifications of oxides recognize discrete molecular species and structures which are polymeric in one or more dimensions leading to chains, layers, and ultimately, to three-dimensional networks. Some typical examples are in Table 14.14 structural details are given elsewhere under each individual element. The type of structure adopted in any particular case depends (obviously) not only on the... 

The structural features of most niobium oxychlorides known to-date are summarized in Table 6.1. The use of a combination of chloride and oxide hgands leads to compounds with unique structure types [41], characterized by a remarkable variety of cluster frameworks, ranging from discrete cluster units to chains, layers, and three-dimensional nets, some topologies of which are unprecedented in compounds containing octahedral clusters. Most of the niobium oxychlorides known to date have anisotropic structures (the exceptions are Cs2LuNb,5Cli70 and PbLusNbsClisOg). 

When coordination polyhedra are connected to chains, layers or a three-dimensional network, this can be expressed by the preceding symbols 2, or 2, respectively. Examples ... 

An open-framework zinc phosphate synthesized under mild hydrothermal conditions possesses two interpenetrating helical channels.414 Piperazine phosphate yields a variety of open framework structures in reaction with zinc, including linear chain, layer, and three-dimensional systems.415... 

Fig. II (a) View of the crystal structure of [Cr(Cp )2][Ni(a-tpdt)2] along [1 —1 0], where it is possible to see three layers of chains aligned either along [1 —1 0] or [1 1 0], cl correspond to the closest D+A intrachain contacts and c2 to the closest A A interchain interlayer contacts, (b) View of two chain layers along the [1 0 2] direction. The chains of the layer on the right are aligned along [1 — 1 0] and on the left along [1 1 0]. c3 corresponds to the closer interchain contacts within the layers...

The metamagnetic behavior of [Fe(Cp )2] [Ni(a-tpdt)2] is attributed to the AF coupling between the FM coupled D+A D+A chains within the chain layers, as predicted from the application of the McConnell I model and spin density calculations, in a similar way to other salts also based on decamethylmetallocenes and other transition-metal bisdichalcogenate complexes with a type I structural motive such as [Mn(Cp )2][M(tdt)2] (M = Ni, Pd, Pt). 

Extreme diversity is exhibited in the coupling of metal halide tetra-hedra to form polymeric halogenocuprate(I) and halogenoargentate(I) ions, there being an abundance of different types of infinite chains, layers, and three-dimensional arrays. Because the object of this article is to focus on variations in metal coordination number, and, in particular, on trends associated with the nature of the cation, structural description will be limited to those types of polymeric anion most frequently encountered hitherto in crystalline halogenocuprates(I) and halogenoargentates(I). 

Silicon, the second most abundant element in the earth s crust, is obtained by reducing silica sand (Si02) with coke. It is purified for use in the semiconductor industry by zone refining. In the silicates, SiC>4 tetrahedra share common O atoms to give silicon oxoanions with ring, chain, layer, and extended three-dimensional structures. In aluminosilicates, such as KAlSi30g, Al3+ replaces some of the Si4+. 

The three fundamental lyotropic liquid crystal structures are depicted in Figure 1. The lamellar structure with bimolecular lipid layers separated by water layers (Figure 1, center) is a relevant model for many biological interfaces. Despite the disorder in the polar region and in the hydrocarbon chain layers, which spectroscopy reveals are close to the liquid states, there is a perfect repetition in the direction perpendicular to the layers. Because of this one-dimensional periodicity, the thicknesses of the lipid and water layers and the cross-section area per lipid molecule can be derived directly from x-ray diffraction data. 

Figure 15. Hydrocarbon chain structure of 2-oleyl-distearin based on x-ray diffraction. The short spacing data indicate that the two types of chain packing (Tu and O n) are attributable to separation of the chains into bilayers with saturated chains and monolayers with unsaturated chains. The long spacing data indicate that the unit layer consists of three chain layers.

Fig. 2 Left. The basic 2D structure in the rod-like liquid crystals made by strongly polar molecules (the dipole moment directions are represented by arrows). The asymmetric rod-like molecules tend to form double layer structures in order to compensate dipole moments. Since the thickness of a double layer is smaller than twice the thickness of a single layer the resulting stress in the system is relieved by the in-layer modulation. Right. 2D structures in polycatenar rod-like liquid crystals. The ribbon like structure is formed by the asymmetric polycatenar molecules. To accommodate the additional alkyl chains layers become slightly bent. Both structures possess the body centered crystallographic unit cell...

Theoretically, since these are layered homologous compounds, a numer-ous/infinite number of compounds are possible in the family However, realistically, we have been able to synthesize pure phases of only the three compounds. Compounds which contained more than four layers of the B12 icosahedral and C-B-C chain layers (which is the case for RB28.5C4) always contained a mixture of other number layers also. In the limit of the boron icosahedra and C-B-C chain layers separating the metal layers reaching infinity (i.e. no rare earth layers) the compound is actually analogous to boron carbide. In the opposite limit, a compound with just one boron icosahedra layer is imaginable. And in actuality, such a MgB9N compound was independently discovered by Mironov et al. (2002). However, such a compound with rare earth atoms has not yet been synthesized. 

As noted in Section 9, the structures of the R-B-C(N) compounds (Figure 21) are homologous to that of boron carbide which exhibits typical p-type characteristics. Boron carbide is the limit where the number of boron icosahedra and C-B-C chain layers separating the metal layers reaches infinity (i.e. no rare earth layers). It has been speculated that the 2 dimensional metal layers of these rare earth R-B-C(N) compounds are playing a role for the unusual n-type behavior, but the mechanism is not yet clear. 

Boron forms a binary carbide, often written B4C but actually non-stoichiometry, and compounds with most metals. The stoichiometries and structures of these solids mostly defy simple interpretation. Many types of chains, layers and polyhedra of boron atoms are found. Simple examples are CaB6 and UB12, containing linked octahedra and icosahedra, respectively. 

Keywords Resistivity, YBa2Cu30rs superconductors, chain-layer, oxygen-redistribution. 

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