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The Weissenberg method

The Weissenberg technique is a development of the rotation technique. A typical camera is shown diagrammatically in Fig. 25. A slotted cylindrical screen is placed between the crystal and the film so that only one cone of reflections can strike the film (see Fig. 26) and then as the crystal is rotated, the film holder is translated parallel to the rotation axis. This results in the reflections (which formed a straight line on the rotation photograph) being spread across the film in a specific 2-dimensional pattern. The screen is movable, so each cone or `layer' of reflections (n = 0, 1, 2 etc.) can be photographed separately. From these photographs can be calculated the lengths of the other crystallographic axes and the interaxial angles. In addition, the symmetry and space group of the unit cell can be deduced from an analysis of the Miller indices of the reflections which are systematically absent (see Fig. 27).


 
Figure 25
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Figure 26
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This is the most common single crystal X-ray camera, and it has been used for the past 40 years for the collection of intensity data for crystal structure elucidation.


 
Figure 27
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\includegraphics {fig27.ps}
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next up previous
Next: The precession method Up: Moving crystal and moving film methods Previous: The rotation method

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