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Sample Spinning

It is normally routine to spin powder diffraction samples, though some flat-plate Bragg-Brentano diffractometers may not have facilities for this. In particular, many diffractometer furnaces do not permit the sample to be rotated at high temperature. Capillary samples are rotated about the capillary axis; flat-plate samples are rotated about an axis normal to the flat plate.

The previous page showed the advantage of spinning the sample, at least for large grained powders where the effects of texture may be reduced (but not eliminated). It is also essential for highly-absorbing or tiny (small quantity) powder samples. One of the drawbacks of measuring a powder diffraction pattern in only one dimension is that one may be unaware of problems due to sample averaging and texture. In this particular aspect, the semi two-dimensional nature of Debye-Scherrer film had an advantage since one could see directly whether the powder rings were homogeneous or spotty. Image plate and CCD detectors when occasionally used for powder work have the same advantage.

With a capillary sample, spinning changes the orientation of the individual grains in the powder sample, with the result that more crystallites will satisfy the Bragg condition some of the time. Since the capillary is fully bathed by the X-ray beam, the number of particles irradiated is unchanged by spinning. By contrast, with a flat-plate sample only part of the sample is illuminated by the beam so that the effect of rotation is to change not only the orientation of the grains, but to irradiate more of the sample.

There is only one serious disadvantage to spinning samples: in Bragg-Brentano geometry the sample may be ejected from the flat-plate sample holder if there is a lack of cohesion between the powder grains. Since sample spillages are not completely avoidable, precautionary action, such as the placing of aluminium foil below the goniometer and sample holder, may be advisable.

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© Copyright 1997-2006.  Birkbeck College, University of London. Author(s): Jeremy Karl Cockcroft