Precession Pattern
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Dialog Box: Precession Pattern [ Diffraction Menu ]

You can select either the original (Old) or transformed (New) atoms.

The Precession angle (mu) and Crystal-Film distance should match those of the real film to which comparison is to be made. The Precession angle determines the extent of the pattern in reciprocal space. If the Fixed Scale option (rather than Fit in Window) is selected, the pattern will be in true scale. This can be altered with the Factor, which is 1.0 by default.

The part of the reciprocal lattice shown is determined by the Horizontal axis, the Other axis and the Level. For example, for a 0kl pattern with b* horizontal, the Horizontal axis is b*, the Other axis is c*, and the Level is 0.

Reflections are shown as squares whose area is proportional to the calculated relative intensity. The intensities are normalized based on the number of electrons in the unit cell ( intensity of reflection 000) and the smallest reciprocal lattice spacing in this net. The Intensity factor scales the edge length, not area, of the squares (thus it really applies to the structure factor).

If an atom has no temperature factor, the B(iso) value in the Default temperature factor edit box will be used. Extinction and absorption are not considered.

Scattering factors used by default are those for neutral atoms from Volume 4 of the International Tables (1974) edition. Atom type numbers must be atomic numbers, or must correlate with the order of the entries in the SCATFAC.DAT file. This file may be edited to supply different scattering factors (see Files Used by CRYSCON). Type numbers are taken modulo 100, that is 6, 106, 206 etc. all indicate carbon. By default number 99 is deuterium (symbol D) and 98 is tritium (symbol T).

If the Show grid box is checked, the reciprocal lattice will be shown as a grid in red on the screen and color output, or as dotted lines in black-and-white output.

The results of this option are shown in a graphics windows (see Types of Windows), and when this window is active, the file menu allows graphics Print and saving Metafiles. There is a different window for the Old structure and the New structure.

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. For example, the space group C-1 is sometimes used. It is possible firstly to transform to the primitive cell (use the standard matrix C->P). However, this would change the a and b reciprocal axes and the indices of the X-ray reflections. Alternatively, the transformation could be from C-1 to P-1 with no change in unit cell. The h+k odd reflections would be calculated and have zero intensity, but the other reflections will be indexed as for the original cell.

If the Show extinctions box is checked, systematically absent reflections are marked with a small circle, in red in color display and output.

Clicking on a reflection will bring up an alert box showing the indices, the d-spacing, the structure factor (|F|), the intensity (I) and the Lorentz-polarization factor.

The font for legends is set in the Window Menu, or the Diffraction Menu of the Control Window.