The
crystal data for at least one material for each zeolite framework type
are available in the database. Wherever possible, data for natural zeolites
or as-synthesized materials (i.e. not calcined) have been included, so
that comparison with an experimental pattern of a new material is easier.
The data on these interactive web pages were first published in the book Collection
of Simulated XRD Powder Patterns for Zeolites (3rd ed.) by M.M.J.
Treacy, J.B. Higgins & R. von Ballmoos. The original data from the
literature were entered into the ZeoFile program
(J. M. Newsam and M. M. J. Treacy, Zeolites, 13,
183-186, 1993).
A note of caution should be addressed to the user of these patterns.
They are very useful in helping to establish the structural purity of
a zeolite phase, yet they may not always allow one to readily and unambiguously
determine the Framework Type of a sample. This assignment is often not
straightforward and may require more sophisticated analyses. A pertinent
review of factors affecting the diffraction characteristics of zeolite
materials has been provided by W. J. Rohrbaugh and E. W. Wu (ACS
Symposium Series411, 279-302,
1989).
In many cases, the materials selected for
inclusion in the database are natural zeolites, whose structures were
refined with single crystal data. However, for many of the framework
types represented, there are no known natural counterparts. In these
instances, typical as-synthesized materials have been selected if a suitable
structure refinement could be found.
In cases where this was not possible, the best available refinement of
the structure of the calcined material was used. It should be borne in
mind that powder patterns generated from such data will show significant
differences in the peak intensities (particularly at low angles) from
the patterns of the corresponding hydrated or as-synthesized materials.
If no reliable structure refinement could be found, coordinates from
a framework geometry optimization (DLS refinement) were used.
In cases where significant differences are observed in the diffraction
patterns of isotypic materials (i.e. those with the same zeolite Framework
Type), several reference materials have been included to illustrate the
extent of the differences observed as a result of variations in composition
and/or symmetry. Outstanding examples are edingtonite and K-F (EDI),
gismondine, amicite, gobbinsite, and Na-P1 (GIS)
, and as synthesized vs calcined ZSM-5 (MFI).
Patterns of the commercially significant X and Y zeolites are also included
to complement the natural faujasite (FAU)
data.
Compositions are expressed in terms of the
full unit cell content. Two compositions are listed: that provided in
the original reference and that calculated from the crystal structure
data.
The powder diffraction data include
the Miller indices (hkl), 2θ value
(for the selected radiation), d-spacing
and relative intensity for each reflection in the 2θ range specified.
The scattering factors used for the framework T- and O-atoms in the
structure factor computations are those selected by the authors of
the original work. If none were specified, atomic (neutral) scattering
factors are used. No adsorption corrections are applied to the data
and anisotropic temperature factors have been converted to isotropic
temperature factors B(iso) Å2.
The powder diffraction patterns are calculated for the radiation and
2θ range set by the user with the program FOCUS written
by Ralf Grosse-Kunstleve (R.W. Grosse-Kunstleve, L.B. McCusker and Ch.
Baerlocher, J. Appl. Cryst.30,
985-995 (1997)). The polarization factor can be changed to reflect the
effect of having a monochromator in the beam or of using highly polarized
X-rays (e.g. synchrotron radiation).
The intensity scale can be specified in different ways to accomodate
extreme situations or to highlight specific regions of the pattern.
The intensity is scaled relative to the height of the peak/reflection
selected. This linear intensity may differ slightly from the integrated
intensity given in the numerical diffraction data.
The powder
diffraction patterns are calculated using a pseudo-Voigt (pV) peak-shape
function
pV = aL +
(1-a)G
where L and G are
the Lorentzian and Gaussian contributions, respectively, and a is
the mixing parameter.
By default, the 2θ step size is
set to 0.02 degrees (i.e. the pattern intensity is calculated for every
0.02 degrees in 2θ), the background level to 0.0, the Lorentz
contribution to 50% (i.e. a = 0.5),
the Full Width at Half Maximum (FWHM) to a constant value of 0.07°,
and the 2θ range of a reflection to 10 times its FWHM. All of
these values can be changed by the user.
The FWHM can be changed by entering the parameters UVW for
the function
FWHM = sqrt(U + V*tanθ + W*tan2θ)
Many real samples will give diffraction
patterns with lines broader than 0.07° as a result of instrumental
broadening, disorder, and/or small crystallite size, whereas synchrotron
measurements might exhibit lines as narrow as 0.02°. For comparison
purposes, it may also be useful to use the V and W parameters
to simulate the 2θ dependence of the FWHM.
For patterns with a high Lorentzian contribution, the peak range factor
might need to be increased to avoid truncation of the peak "tails".
Images
of the zeolite powder patterns are generated as 8 x 6 inch pictures
with three different resolutions: 50 dpi, 80 dpi and 110 dpi. The resulting
images correspond to the following screen and file sizes.
