Zeolite materials with disordered framework structures cannot be described properly within the layout of the
Database of Zeolite Structures,
because that layout assumes that the framework structures are perfectly ordered in 3 dimensions. In order to address this problem, this second database has been created and designed specifically to describe such materials. They are not assigned 3-letter codes, but are grouped into
Intergrowth Families that share a common Periodic Building Unit (
PerBU) that can be connected in different ways.
The first disordered zeolite that was included in the
Atlas of Zeolite Framework Types was zeolite Beta. Although its framework structure is not completely ordered, the material was considered to be so important to the zeolite community that it needed to be included in the
Atlas. To indicate that it was a special case, an asterisk was added to its 3-letter code (
*BEA). Over the years more zeolites with disordered framework structures were discovered and described, and some of these were also added to the zeolite frameworks in the
Atlas and later to the
Database of Zeolite Structures. The asterisk was initially defined to indicate a polytypic framework structure (i.e. one with a single layer that could be stacked in different ways). As time went by, however, this strict definition was relaxed to include other types of disorder.
Disordered zeolites are usually described in terms of a series of end-member polymorphs. In the mineralogical world, polymorphs are materials with the same chemical composition but different crystal structures (e.g. graphite and diamond or zinc blende and wurzite). In principle, all zeolites and minerals with the composition SiO
2 are polymorphs. To make the term more useful in the zeolite world, a slightly different definition is used. A zeolite polymorph results from the connection of the same well-defined, ordered structural unit(s) (e.g. a layer) to produce different 3-dimensional frameworks with different T-atom connectivities. End members are those with a periodicity in the third dimension. The collection of the different polymorphs created from a single structural unit (a periodic building unit or
PerBU) are called a
Family and the family name is derived from the material name. Thus we speak of the Beta family, which encompasses all the different polmorphs of zeolite Beta. As in the case of a
Framework Type for zeolites with fully ordered structures, the chemical composition of the framework is not considered.
The specific
PerBU for a material is not always easy to define. It should be emphasized here
that the
PerBU used may just be a convenient choice to descibe a disordered material in a
way that is easy tounderstand.
It does not imply that this is necessarily the growth unit.
A disordered structure is included in this database only when there is a least some experimental evidence that it actually exists. Some hypothetical disordered structures were compiled by H. Gies and H. van Koningsveld in 1999, and links to those pdf files are provided at the bottom of the entry page to the
Intgergrowth Families.
Classification of disorder types
1-dimensional stacking disorder
Single layer
This is the classical type of stacking disorder with a 2-dimensional
PerBU (i.e. a layer) that can be stacked along the third dimension in different ways. Zeolite Beta and polytypic zeolites belong to this class of disorder.
ABC-6 clan
The ABC-6 zeolites are a subclass of this type of disorder, and can be described in terms of a stacking of layers of hexagonally arranged 6-rings. The three possible positions of the layer are designated using the letters A, B and C, as in the closest packing of spheres. Although these layers of 6-rings do not form a connected layer, the 6-rings themselves are on the same level, and indeed 26 of the ordered zeolite framework structures can be described using this nomenclature.
In principle, a large number of disordered stackings are possible in the ABC-6 clan and many have been reported, but only a limited number have been investigated in detail. Therefore, only a selection of the possible intergrowths, for which there is experimental evidence, are presented.
2-dimensional disorder
If the
PerBU itself or a small layer-connecting unit is only periodic in one dimension (i.e. a row or chain rather than a layer), disorder in two dimensions is possible.
Currently only examples with layers connected via arrangements of small units (e.g.
d4
r) that are ordered in only one dimension, are listed.
Other disorder
This database is still being developed and not all known disordered zeolites have been
classified and/or described. These materials are listed here with a reference to the original literature. With time, they will find their place either in one of the categories listed above or in a new one.
Choice of PerBUs
As mentioned above, the choice of a
PerBU for a particular family can be somewhat arbitrary. Generally, the description given in the literature is used. However, especially in the case of older literature, there may now be a different understanding of the disorder or just a different, perhaps simpler way, of looking at it. The ambiguity comes from the fact that it is not always easy to determine what the true disorder mechanism is.
In this database we have tried to select a
PerBU that allows a structure to be described in a manner that is consistent with that used for all similar disorder types. The hope is that this will make it easier
for the user to understand the disorder, and that common features in the way disorder occurs can be recognized more easily. The latter may help to improve our understanding of disorder in zeolites.
The selected
PerBU is shown at the top of each
Familly page in the form of a 3D display along with some information specific to this
PerBU.
For simplicity, only the T atoms are shown, except for interrupted frameworks or where the positions of the linking O atoms are important.
Connectivity pattern
The possible connectivity patterns (the way the
PerBUs can be connected to one another), are described in a table. For each connectivity pattern, an image that shows the connection in three different projections is provided. As for the
PerBUs, only the T atoms are shown for simplicity. Wherever possible, the disorder is described as a translation of a
PerBU, because translations are much easier to visualize than are rotations. However, in some cases the
PerBU has to be mirrored
(or inverted) before it can be translated and connected.
Each connectivity pattern is assigned a color, which is then used in the 3D displays in the following section. With the exception of the ABC-6 zeolites, where the A, B and C layers are defined in terms of the first layer, the connection is always defined with respect to the previous layer. Where appropriate, additional information and explanations are given for the connectivity patterns below the table.
Simple ordered end members
A combination of the different connectivity patterns leads to the disorder in a particular material. To illustrate these different connections in three dimensions, a few combinations are used to form some simple periodic end-member structures. The geometry of each of these structures has been optimized using the distance least-squares program
DLS-76 (Ch. Baerlocher, A. Hepp and W.M. Meier, ETH Zurich, Switzerland (1977)), and is presented with some crystallographic details and a colored-coded 3D display. The color coding indicating the type of connection should make it easier to recognize the
PerBUs and their connectivity patterns within these simple polymorphs. The corresponding cif file can be downloaded using the
CIF pull-down menu. As for the
Framework Types in the
Database of Zeolite Structures, an SiO
2 composition and the highest possible symmetry is assumed, and the unit cell parameters are included in the optimization.
The end-member polymorphs are labelled somewhat arbitrarily with capital letters. If they have already
been described in the literature, the labelling is selected accordingly, or the equivalence is noted. In most cases, the polymorphs are hypothetical and have not been observed as pure materials, and this is noted in the description. When the polymorph does indeed exist and has been assigned a 3-letter code, a link to that structure in the
Database of Zeolite Structures is given.
At the end of this web page a short segment of a disordered structure is provided as an example.
Observed X-ray powder diffraction patterns
When available, one or more experimentally measured X-ray powder diffraction patterns of materials within a particular Intergrowth Family are provided. If possible, the diffraction pattern of a calcined material is included, because that will be more useful for comparison with the simulated DIFFaX patterns (see below).
Simulated DIFFaX patterns
For zeolites with 1-dimensional disorder, it is possible to simulate X-ray powder diffraction patterns for statistically mixed combinations of end-member polymorphs using the program
DIFFaX (M.M.J. Treacy, J.M. Newsam and M.W. Deem,
Proc. Royal Soc. London A,
433, 499-520 (1991)). Such patterns have been generated for most of the
Intergrowth Families with 1-dimensional disorder.
For the simulations, an all-silica composition is assumed, and the framework structures of the end-member polymorphs are optimized using
DLS-76. A disorder of end-member polymorphs rather than of the
PerBUs themselves is used for the calculations. When possible, an experimentally determined unit cell and/or symmetry is taken into account.
The simulation is done by calculating an orthorhombic version of the structure of each end member with the stacking of the layers oriented along the
z direction. These initial structures of the end members that are to be combined are calculated to have the same number of layers. For example, a polymorph with all layers shifted by 1/3
a will require three layers to get back to the position of the original layer, while one with the layers shifted by +1/3
a alternating with -1/3
a will only require two. In this case, six layers are calculated for each polymorph. These structures are then mixed in a random manner for a series of compositions, starting with 100% of one polymorph and then adding the second one in 10% increments up to 100% of the second polymorph.
These series allow a user to estimate the polymorph composition of a specific material within an
Intergrowth Family. The user can upload an experimental diffraction pattern, which can then be compared with these simulated patterns in an interactive manner. The wavelength of the simulated pattern can be adjusted to match that of the experimental one.
It should be noted that these simulations are based on the framework structure alone (i.e. no non-framework species are included) and assumptions regarding the symmetry and the unit cell dimensions can affect the patterns considerably. They should only serve as a starting point for a more detailed investigation.
last updated: 4-August-2023