Cation positions model

Fig. 20.22 Zn F ion stabilized in the cation position modeled by four-membered ring...

A related system is that of the titanomaghemites, Fe2Ti05, which are formed by oxidation of the titanomagnetites. These are spinels with vacancies in some of the cation positions. The detailed distribution of cations and vacancies is not fully understood a discussion of the different models and possibilities is given by Lindsley (1976). A non-linear relationship between the unit cell size of titanomaghemite (a = 0.8483 nm) and that of maghemite (a = 0.835 nm) was matched by a non-linear increase in the Curie temperature from 80 to 450 °C (Dunlop Ozdemir, 1997). 

Consideration of the framework models strongly suggests that not only are the pore systems different but it is likely that the cation positions will be different. A possible consequence would be an alteration of ion exchange or sorption selectivities. 

The electron sea model (see Figure 6.2) for metal bonding proposes a theory that explains observed metal properties. In this model, we can envision that metal bonds are formed when a uniform array of metal cations, positively charged metal ions, are surrounded by a sea of electrons. 

The optimized sodium cation positions in a six-ring of FAU zeolite structure containing two A1 atoms in para-position (denoted as Na-Al-2p) ° is shown in Figure 1. This position of the cation is representative for Sn cation position in Y and X zeolites," as well as the position of Na in Na-EMT zeolite. As expected, for this and the other zeolite model structures, Na" prefers positions near to oxygen centers bonded to A1 atoms rather than those of Si-O-Si bridges. Also, tbe cation is far from oxygen centers which are connected to compensating cations, an additional proton in this case. 

The large positive shift and the parabolic behaviour of the 5 = f(N) curves in the case of divalent cations was attributed first by Fraissard at al. [2] to the high polarisability of xenon and the distortion of the xenon electron cloud by the strong electric fields created by the 2+ cations. Later, Cheung et al. [5] proposed a model to explain the strong adsorption of xenon in zeolites with 2+ cations (Ca2+, Mg2+, Ba2+). It consists in extending the electron attraction described above to the point where an electron is transferred from the xenon to the cation. This model suggests that a partial bond between the xenon atom and the 2+ cation is formed by donation of a xenon 5p electron to the empty s-orbital of the 2+ cation. 

The location of extra framework cations is a major problem in characterising zeolites. Simulation is becoming an increasingly powerful tool for the exploration and rationalisation of cation positions, since it not only allows atomic level models to be compared to bulk experimental behaviour, but can also make predictions about the behaviour of systems not readily accessible to experimental probing. In the first part of this work we use the Mott-Littleton method in conjunction with empirical potential energy functions to predict and explore the locations of calcium cations in chabazite. Subsequently, we have used periodic non-local density functional calculations to validate these results for some cases. 

Image simulation of the layer structure was carried out on the basis of a cation-disorder model for the imaging conditions of Fig. 13.29. In this model the cation positions of the YBa2Cu307 structure are partially occupied by different atoms corresponding to the measured stoichiometry. The result is displayed in Fig. 13.30. We see a pattern similar to the experimental one. The irregularities in the experimental image can be attributed to local ordering of the cations. 

Because they-th and the (/+l)-th TS layers must connect two packets p2y and q2y+i with the same orientation parity (to preserve the octahedral coordination of the M cations), there are only eight possible pairs of TS unit layers (DD D D TT T T DT D T TD T D ). In addition, to match the cation positions, the layer stacked over a D or T layer must be shifted by -a/3, whereas the layer stacked over a D or T layer must be shifted by +a/3. Within the Pauling model only the octahedral cations have different coordinates in the four TS unit layers. However, their contribution to the layer Fourier transform becomes identical when the following conditions are satisfied ... 

In recent years, modem methods of theoretical chemistry were used to address the problem of the stmcture of Y-AI2O3. Digne et al. (158,159) and Krokidis et al. (119) proposed a stmcture model characterized by an ortho-rhombicaUy distorted ccp oxide lattice and nonspinel cationic positions (space group P2 /m, Z=8), with 25% of the Al ions in tetrahedral... 

Figure 3.3 Rietveld refinement (top) of the structure of the dehydrated sodium form of the gallosilicate TNU-7 against synchrotron X-ray powder diffraction data, together with a view of the final structure, in which the sodium cation positions are indicated (bottom). An initial model for the framework was obtained from a description of the poorly ordered aluminosilicate ECR-1 cation positions were located from difference Fourier analyses and their positions and occupancies refined. The plot shows the observed data, the calculated profile and a difference plot. The final match is an acceptable fit (Rwp = 6.5%). Electron diffraction (middle) along the [001] and [010] zone axes shows that this structure is a strictly alternating intergrowth of MOR and MAZ layers (see Figure 2.11) because no evidence of streaking in the diffraction maxima (characteristic of disorder) is observed. [Reproduced from reference 30 with permission. Copyright 2006 American Chemical Society.]...

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