Framework distortion

Fig. 3.—The structure of trisilver cobalticyanide. There are three mutually interpenetrating rhombohedral frameworks, distorted from the cubic frameworks of Fig. 2 in such a way as to achieve increased density and closer approximation to the optimal packing of the bulky cobalticyanide groups.

Schneider and co-workers (159) attempted to rescue the Grant-Cheney approach, at least for methyl substituents. They refined it by using a modified potential for the repulsive force (186) along with fully relaxed molecular structures as determined by force-field calculations. This indeed led to a satisfactory correlation of yg-SCS(CH3) with repulsive interactions in methylcyclohexane and some methylbicyclo[2.2.1]heptanes (159). Schneider s treatment, however, implies that a signal shift originates not only from steric H-H repulsion but includes effects from secondary carbon-framework distortions (85,159). 

The similarities in the response of the structure to decreased temperature and to decreased substitutional cation size is apparent in the matching ferroelastic transitions from cubic to tetrag-onal crystal systems. Moreover, Palmer et al. (1997) report that the style of framework distortion associated with decreased temperature is identical to that produced by the replacement of Cs by Rb and K (Fig. 4) both cooling and small-cation substitution provoke a twisting of tetragonal prisms about [001]. Because the larger cations prop open the framework and limit the... 

A structure similar to that of LaY zeolites has been suggested for CeY zeolites (3,7). However, structural x-ray studies of CeX zeolites have shown (9) that the presence of trivalent and especially tetravalent- cerium results in a considerable distortion of the 6-membered ring. Such a distortion could explain the lower hydrothermal stability and stronger dealumination of CeY zeolites vs LaY zeolites. Indeed, the loss in crystallinity and surface area observed for steamed CeY zeolites as compared to corresponding LaY zeolites (Table I) is indicative of significant structural collapse. Furthermore, the paramagnetism of trivalent cerium ions and the local framework distortions, caused primarily by tetravalent cerium ions, will result in a deterioration of spectral resolution. 

The effect of cation movement and framework distortion on the infrared pattern of a Ca-exchanged Y zeolite (Si/Al = 2.5) is shown in Figure 12. Dehydration and complete dehydroxylation of zeolites with similar cation composition and framework topology (e.g., Ca-exchanged... 

Despite its simplicity, the tolerance factor has reasonable predictive power, especially for oxides, where ionic radii are known with greatest precision. Ideally t should be equal to 1.0 and it has been found empirically that if t lies in the approximate range 0.9-1.0 a cubic perovskite structure is a reasonable possibility. If t>l, that is, large A and small B, a hexagonal packing of the AXj layers is preferred and hexagonal phases of the BaNiOj type form (Chapter 3). In cases where t of the order of 0.71-0.9, the structure, particularly the octahedral framework, distorts to close down the cuboctahedral coordination polyhedron, which results in a crystal structure of lower symmetry than cubic. For even lower values of t, the A and B cations are of similar size and are associated with the ilmenite, FeTiOj, structure or the C-type rare earth Ln Oj stmcture. 

The structure-types 44 and 47 are similar to the mineral chabazite. However, while the symmetry of chabazite is rhombohedral, for both the 44 and 47 materials framework distortion causes a reduction in symmetry from rhombohedral to triclinic (33). 

B. Baumgartner, G. Muller Framework distortions by large ions in MAlSi20e alumino-silicates with keatite structure , Eur. J. Mineral. 2, 155-162 (1990)... 

A different type of the framework distortion, leading to the shrinkage of pores, was observed in... 

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