Long-range Cation Order-disorder

The cell dimensions of zeolites are determined by the total system, which includes cations and water molecules. Even if resolution of the differences in experimental data confirms the existence of discontinuities, the possibility of effects resulting from resiting of cations and molecules must be considered as well as Al,Si order-disorder. Certainly the X-zeolites studied by Olson (47) have long-range T order, but for faujasite and Y-zeolites the evidence from cell dimensions is equivocal. Ordered crystals and most natural minerals have undergone more annealing than synthetic specimens. The present data are consistent with the possibility that natural faujasite is more ordered than synthetic Y zeolite, but the evidence is inconclusive. 

LDHs indicate a preference for ordered layers and that if the layers, as well as the interlayers, were disordered, long range ordered stacking of the layers in two- or three-layer polytypes would be unlikely to occur. Two possible ordered arrangements of cations with ratios of 3 are shown in... 

This model does not cover energetic aspects, especially at the disorder-order transition. Only electrostatic considerations concerning the cation coordinations have been examined, which explains the slight differences observed for y > 0.25. Moreover, our assumption of the range of disorder seems reasonable since it describes the observed phenomena. Finally, one understands better why, when the long-range ordering disappears close to y = 0.25, the number of tetrahedra is smaller than it would be if the structure was still ordered (m = °°, Fig. 16). 

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