Powder Diffraction
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Dialog Box - called from: Display Menu

To calculate diffraction, the type numbers for atoms (Atom Data dialog) must be valid atomic numbers, or those numbers modulo 100, or must anyway correlate with the table of scattering factors in the SCATFAC.DAT file if you modify that file.

If you select a List, the list will appear in a Text window (see Types of Windows). There will be a line in the listing for each crystal form, or symmetry-unique set of reflections (with center of inversion assumed). If there is more than one wavelength, the 2-theta and relative intensity will be listed for each in that line.

If you select a Graph, you may show either Bars or a Curve - the curve will use Gaussian lines with the selected full width at half maximum (FWHM). For a curve, the location of individual reflections will be shown as a short bar underneath the curve.

The Line width pertains to the width of all lines or "pen strokes" in the graph, not the width of each spectral peak (FWHM).

If the Vertical scale factor or magnification is 1.0, the strongest peak will just fill the plot area. If you want to see more detail in the weaker reflections, this factor can be increased and the stronger reflections will be truncated.

If the atom records include temperature factors, they will be used. If an atom has no temperature factor, the B isotropic value in the Default temperature factor box will be used. No absorption or extinction corrections are applied.

There are four options for wavelength; averaged K(alpha); separate K(alpha)1 and K(alpha)2; averaged K(alpha) and (separate) K(beta); and a single Specified wavelength. For the first three options, the X-ray tube target element must be selected in the Radiation combo box. When two wavelengths are calculated, the bars indicating the second (K(alpha)2 or K(beta)) are shown in red on the screen and in color output, and as a dotted line in black-and-white output.

In the case of K(alpha) and K(beta) the relative intensity of K(beta) must be specified (Beta inten.). For unfiltered radiation this is in the neighborhood of 0.2. Of course it will be smaller for filtered radiation.

This option gives only a semi-quantitative calculation of powder intensity, intended for purposes of identification. For somewhat more elaborate options, including the ability to show multiple species, try the program Powder Cell. For highly quantitative reproduction of powder patterns, there are now many Rietveld programs available (see Import files).

The results of the Graph option are shown in a graphics windows (see Types of Windows), and when this window is active, the File menu allows graphics Printing, Raster Files and Metafiles or PICT files.

For this option to work properly the space-group symbol entered either in the Space Group from Table or Custom Symmetry option must be valid for a standard setting. If it is not, it should be possible to transform to a valid setting using the program CRYSCON (Shape Software).

NOTE: Many of the sample files provided with ATOMS, especially the Basic Inorganic structures, are idealized structure types. They may have a unit cell scaled to a bond length of 1.0, or use arbitrary numbers for types instead of atomic numbers as required for diffraction. Therefore they will not always give correct diffraction results.