Octahedral tunnel structures

Octahedral tunnel structures. Metal-oxygen octahedra, BO, form host lattices characterized by large tunnels where cations are located. Thus, perovskites form a large family which are described by four-sided tunnels. There are a large number of tunnel structures which can be classified according to the size and the shape of the tunnels. Tunnel structures with angles of 90° or 60 -120° occur commonly. We shall briefiy examine the general features of tunnel structures. (Note that the bronzes we discussed earlier are tunnel structures). 

An efficient oxidation catalyst, OMS-1 (octahedral mol. sieve), was prepared by microwave heating of a family of layered and tunnel-structured manganese oxide materials. These materials are known to interact strongly with microwave radiation, and thus pronounced effects on the microstructure were expected. Their catalytic activity was tested in the oxidative dehydrogenation of ethylbenzene to styrene [25]. 

The remaining compounds listed in Table II all adopt structures with infinite metal-metal bonded chains consisting of octahedral cluster units fused on opposite edges. However, because of the large difference in effective ionic radius of the cations concerned, very different lattice types are dictated. The compounds NaMoi 06 (19,22) and Bas(Moit06)8 (17) adopt tunnel structures with the Na+ or Ba2+ ions located in sites along the tunnels with 8-fold coordination by oxygen atoms. 

FIGURE 4 (a) [100] projection of the palygorskite structure and (b) an extended arrangement showing the tunnel structure. Sepiolite is very similar, except there are 8 Mg in the octahedral positions of each main block, rather than the 5 Mg, A1 of palygorskite. (4a is reproduced with the permission of the Mineralogical Society of Great Britain and Ireland from Ref 13.)... 

The crystal structure of pyrolusite, or f -Mn02, is the simplest one within the family of compounds with tunnel structures. The manganese atoms occupy half of the octahedral voids in the hexagonal close packing of oxygen atoms in an ordered manner, thus forming a rutile-type structure. The distorted MnOg units build... 

Fig. 2.6 Structure of akaganeite. a) Arrangement of octahedral double chains in tunnels with chloride ions in the centre ofthe tunnels, b) Ball-and-stick model with unit cell outlined.

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