Cation configuration

HREM images of the different members of the family are obtained, particularly along the [100] zone, which is the most instructive one the cation configuration can be readily identified from symmetry considerations. There is clearly a one to one correspondence between the white dot configuration of the... 

Zeolites aie crystalline. Diffraction methods ate therefore the prime technique for structural characterization. Local stmctural features (such as Brpnsted add sites, non-frramewcHlr cation configurations etc.) that do not obey the full crystallographic symmetry are often present These contribute to the diffraction pattern only wealdy if they are dilute, or they t pear as disorder in the diffraction-averaged stracture. Such features must be probed by techniques such as solid state nmr or EXAFS that are sensitive to local enviromnent, but proper interpretations of such data must still be made in the context of the crystal structure provided by the di raction results. 

In combination with this increased knowledge of surface chemistry, a detailed knowledge of the material bulk is essential. It is clear from the discussion in this entry that the target materials set will be a multicomponent oxide with potentially complex cation and anion lattice structures. When material properties depend upon cation configuration, e.g., the double perovskites, or oxygen stoichiometry, these properties must be accurately determined in a working cell. 

When metal ions fill in the vertex of tetrahedral and octahedral, there may be two cases of boundary. One is the normal feature of the cation distribution. At this case, all the divalent ions are located at A site, for example, ZnFe204. The second situation is that ions with various valences occupy octahedral vertex. Magnetite has an abnormal spinel-t3 e structure, and also has mixed cation configuration. 

Fig. 5.64 The formation of Magneli phases (left). Oxygen (dark circles) is removed at low oxygen partial pressure, and ordered interstitial cation configurations are formed (pale circles). The thermodynamic stability range of the resulting phases is shown in the graph of the equilibrium oxygen partial pressure as a function of composition (right) (cf. Chapter 4). According to Refs. [6,216].

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