Layer next nearest

EXAFS data at the Co and Al K edges for Co Al - Cl LDHs were consistent with an ordered array of cations for n = 2,3 and 4. In each case the Al has a second coordination shell of 6 Co ions and the Co has a second shell of 6/n Al and (6-6/ ) Co next nearest neighbors [46]. The peaks arising from the focusing effect are still observed in the Co K edge EXAFS of the material showing that the cobalt cations in the layers remain ahgned. 

There is an ill-defined boundary between molecular and polymeric covalent substances. It is often possible to recognise discrete molecules in a solid-state structure, but closer scrutiny may reveal intermolecular attractions which are rather stronger than would be consistent with Van der Waals interactions. For example, in crystalline iodine each I atom has as its nearest neighbour another I atom at a distance of 272 pm, a little longer than the I-I distance in the gas-phase molecule (267 pm). However, each I atom has two next-nearest neighbours at 350 and 397 pm. The Van der Waals radius of the I atom is about 215 pm at 430 pm, the optimum balance is struck between the London attraction between two I atoms and their mutual repulsion, in the absence of any other source of bonding. There is therefore some reason to believe that the intermolecular interaction amounts to a degree of polymerisation, and the structure can be viewed as a two-dimensional layer lattice. The shortest I-I distance between layers is 427 pm, consistent with the Van der Waals radius. Elemental iodine behaves in most respects - in its volatility and solubility, for example - as a molecular solid, but it does exhibit incipient metallic properties. 

Theoretical and experimental approaches of the framework Si/Al ratio effect on the acidity led to the conclusion that the strength of the protonic sites of zeolites is influenced by the presence of neighbors (11, 12). Each framework Al atom has 4 Si atoms (Lowenstein s rule) in the first surrounding layer (nearest neighbors) and, depending on the zeolite topology, 9-12 Al or Si atoms in the second layer (Next-... 

In AlBi the B atoms are arranged in layers with layers of A1 interleaved between them. The structure of a B layer is the same as that of a layer in the graphite structure (p. 734). Each B is here equidistant from three other B atoms (at T73 A), the next nearest neighbours being a set of six A1 at the vertices of a trigonal prism (Fig. 24.3(b)). Most of the layer borides have the AlBj structure, but in the Mo-B and W—B systems there are also phases with the ideal composition M Bs,... 

The structures of these elements are described in earlier chapters. The graphite structure is obviously exceptional, and the heavier elements in the subgroups IVB-VIB become increasingly metallic. In IVB Sn also forms the more metallic white polymorph, and Pb is a metal. In VB there is a progressive decrease in the difference between the distances to the 3 nearest and 3 next nearest neighbours in the layer structures of As, Sb, and Bi, while in VIB the interatomic distances between the chains in Se and Te indicate interactions between the chains stronger... 

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