Crystallographically different site

Highly selective exchange as related to the presence of crystallographically different site groups... 

A first generalization of the Chudley-Elliott model has been developed in order to deal with crystallographically different hydrogen sites. Crystallographically different sites occur, e.g., for H diffusion over the tetrahedral interstices in b.c.c. metals like Nb. In this case the hydrogen sublattice consists of six superimposed b.c.c. 

CeiPdioSh and Ce Pdi Gcf,. These materials are among the heaviest HF compounds known, with y 8J/(molK ). They crystallize in a cubic phase with the Cr23Cfi superstructure where the Ce atoms occupy two crystallographic different sites denoted Ce-1 and Ce-2 (Takeda et al. 1995). Brief conference reports on p,SR studies on the two materials were recently presented. 

The partial oxidation of propylene occurs via a similar mechanism, although the surface structure of the bismuth-molybdenum oxide is much more complicated than in Fig. 9.17. As Fig. 9.18 shows, crystallographically different oxygen atoms play different roles. Bridging O atoms between Bi and Mo are believed to be responsible for C-H activation and H abstraction from the methyl group, after which the propylene adsorbs in the form of an allyl group (H2C=CH-CH2). This is most likely the rate-determining step of the mechanism. Terminal O atoms bound to Mo are considered to be those that insert in the hydrocarbon. Sites located on bismuth activate and dissociate the O2 which fills the vacancies left in the coordination of molybdenum after acrolein desorption. 

The Ti4+ distribution in TS-1 has also been studied by computational methods (34,62,160-163). The actual location of the Ti atoms in the framework of titanosilicates is difficult to determine experimentally because of the low Ti content (Section II), and information obtained from theoretical methods is, therefore, of considerable interest. In the orthorhombic MFI structure, substitution can take place at 12 crystallographically different tetrahedral (T) sites (T1-T12) (Fig. 1 and Section II.A.l.b). In the monoclinic MFI framework, the mirror symmetry is lost and 24 crystallographically different T sites can be distinguished (Fig. 31) (160). 

Fig. 31. Crystallographically different T-sites in MFI. T1 (T2,...,T12) and T13 (T14,...,T24) are related by a mirror plane in orthorhombic MFI [Reprinted from Njo et al. (160) with permission. Copyright (1997) American Chemical Society].

In some cases, mixed valence states may appear simultaneously on two different atoms because of an overlap of the redox energies associated with the two atoms. In the ilmenite FeTi03, the valence states Fe " and Ti are stabilized with an ordering of Fe and Ti on crystallographically inequivalent sites FeV03, on the other hand, has the corundum structure with Fe and V randomly distributed on equivalent sites with the valence states Fe and This observation places the top of the Fe " 3d band (level) below the bottom of the Ti 3 d band, but above the top of the 3 d band, so charge transfers of the type... 

The parameter ij measures the crystallographic difference between the M11 and M1V sites. PES studies show that the electronic difference also becomes less as ri increases.144 If X came to the midpoint between Mn and M111 (i.e. rj = 1) then all the M ions would become equivalent and a similar situation to the Krogmann salts might be expected, with very high conductivities. The position of the intervalence band is also strongly dependent on rj and moves to lower frequencies as rj approaches unity.145... 

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