Stacking disorder

A) A stacking disorder (De Wolff disorder intergrowth of ramsdellite- and pyrolusite- type units, as already described above). This kind of disorder can be quantified by two parameters ... 

Zeolite structures sometimes remain unsolved for a long time, because of either their complexity, the minute size of the crystallites or the presence of defects or impurities. One extreme example of stacking disorder is provided by zeolite beta [1,2], Different stacking sequences give rise to two polymorphs (A and B) in zeolite beta that always coexist in very small domains in the same crystal. Not only do the small domains make the peaks in the powder X-ray diffraction pattern broad and thereby exacerbate the reflection overlap problem, but the presence of stacking faults also gives rise to other features in the diffraction pattern that further complicate structure solution. 

A. Baronet, Polytypism and Stacking Disorder, in Reviews in Mineralogy, Vol. 27, P. R. Busek, Ed., Mineralogical Society of America, Washington, DC, 1992, Chapter 7. 

Dinaphtho(7, l -l,13)(l",7"-6,8)peropyrene (89) exists in two crystalline modifications, a and ]8. A detailed study of the a-form (Robertson and Trotter, 1959a) shows that there are four molecules in the unit cell and that the space group is C2. This would imply that no molecular symmetry is utilized in the crystal structure, but this is not so. A detailed examination of the X-ray photographs revealed the presence of a stacking disorder and it transpires that the asymmetric... 

Before the similar situation in cylindrite can be treated, we have to describe the special type of layer stacking disorder which is inherent to non-commensurate layer structures. When two types of layers are semi-commensurate (or nearly so) and non-modulated in the semi-commensurate intralayer direction (e.g. b), the layer B can be placed on layer A in a number of ways with equivalent layer match, mutually displaced by the translation mb for the primitive mesh A, or mbAl2 + c tl in the more usual case of a centred layer mesh m stays within the range of the vernier repeat (n) of the layer A. Several of these positions will not coincide with each other, depending on the ratio of to ng in the semi-commensurate direction The same will in general be true for the positions of layer A on layer B, but this time the positions will be determined by the vectors of the layer B. 

Hutchison, J. L., McLaren, A. C. (1976). Two-dimensional lattice images of stacking disorder in wollastonite. Contrib. Mineral. Petrol., 55, 303-9. 

Muller, W. F. (1976). On stacking disorder and polytypism in pectolite and seran-dite. Z. Krist., 144, 401-8. 

To quantify the amount of the resulting ice phase from diffraction data a proper description of these defective ice phases is needed. Here we present a way to fit the diffraction peak profiles with different models of stacking disorder. This may allow us to distinguish different samples with different histories in terms of their stacking distributions. An adapted range of sufficiently large unit cells with different, non-random stacking sequences is created for each sample and a linear combination of these replicas is fitted to the measured neutron diffraction data. In this way a quantitative determination of the total amount of ice present can be obtained. 

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