Intergrowth effects

Hanif, N., Anderson, M.W., Alfredsson, V., and Terasaki, 0. (2000) The effect of stirring on the synthesis of intergrowths of zeolite Y polymorphs. Phys. Chem. Chem. Phys., 1, 3349-3357. 

The discovery of thallium containing superconductors (4) was another important development. Several superconducting phases exist and consist of intergrowths of rock salt (TI-O) and perovskite layers. They have been reported with zero resistance and Meissner effect up to 125K, i.e., with the highest critical temperatures discovered so far. 

Other than in their mean compositions. Slow cooling produces relatively large crystals of each phase, while fast cooling produces close intergrowths textures are discussed further in Section 4.2.1. Zoning occurs readily in the ferrite (Section 2.3.1), and may also occur in the aluminate. The distribution of atoms between octahedral and tetrahedral sites in the ferrite depends on the temperature at which internal equilibrium within the crystal has been achieved (Section 1.5.1). The degree of crystallinity of both phases appears to vary with cooling rate (V3). All these effects, and perhaps others, may affect the behaviour of the interstitial material on hydration. 

Values of n intermediate between the above small integral values are common, as we have already indicated indeed the structures n = 5 and n = 8 have not been obtained in a completely pure state. Intermediate structures may be approximated to combinations of two adjacent integral n types, = r ni+ s n. where 2 — i = 1, and i, 2> and s are integers. But (contrary to earlier assumptions) they are not simple, regular intergrowths. Instead, the vernier effect is spread evenly over half the length of the unit cell, b/2 = (r f>i -J- 5 b tl, which may be very long values up to more than 150 A have... 

Another interesting observation of the earth grown crystals is that nucleation only occurs on the membrane or on crystals that are attached to the membrane. This may have to do with intergrowth and twinning of the original crystals. In time, some of the crystals fall to the bottom of the reactor due to gravitational effects. 

Such an effect is understandable in view of the distinction between erionite and offretite structures published by Bennett and Card (2, 9). The designated lines are forbidden for the offretite structure. Card has examined our synthetic erionite product by electron diffraction and found disordered intergrowth with widely varying proportions of erionite and offretite structures (8). 

The difference is more notable in n-octane adsorption which is shown in the last 2 columns of Table II. Zeolite A shows substantially the same capacity and adsorption rate for n-octane as for n-hexane. But for erionite, both natural and synthetic, n-octane capacities, and particularly the adsorption rates are substantially reduced. Here the difference between synthetic and natural erionite adsorption rate is quite large. It is possible that this is an effect of residual cations. However, simple exchange of Na" and for H" showed little change. We believe the more probable explanation is the intergrowth of offretite in the erionite crystal. The large offretite channels could give more rapid distribution of the sorbate molecule within the synthetic erionite crystal. 

The figure of merit should change dramatically between composition phases. Moreover, within each phase a, the figure of merit should also vary with y = X - Xo, due to crystallinity effects such as crystallite size, intergrowths, defects, and faulting (van Dover et aL, 1998). In addition, the noncomposition variables should also affect the measured figure of merit. The noncomposition variables are denoted by the fi-dimensional vector z, with... 

Fig. 3. Illustration of the effects of multiple scattering in an image of an intergrowth bismuth-tungsten oxide bronze, nominally Bio.iWOa. The simulated images shown correspond to thicknesses of 20a (near the crystal edge) and 60a respectively, with an underfocus of 1220a.

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