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Powder diffraction

A powder is a polycrystalline material in which there are all possible orientations of the crystals so that similar planes in different crystals will scatter in different directions.

Diagram of powder scattering

Scattering in X-ray powder diffraction

In single crystal X-ray diffraction there is only one orientation. This means that for a given wavelength and sample setting relatively few reflections can be measured: possibly zero, one, two (as in the video) or possibly up to say three or four. As other crystals are added with slightly different orientations, several diffraction spots appear at the same 2θ value and spots start to appear at other values of 2θ. Rings consisting of spots (spotty rings) and then rings of even intensity are formed. A powder pattern consists of rings in 2-dimensions, and spheres in 3-dimensions, of even intensity from each accessible reflection at the 2θ angle defined by Bragg's Law.

The other situation which is intermediate between single crystal and powder diffraction is when the sample is oriented and the spots are spread into arcs. This is covered later in the TLP.

This animation shows the relationship between single crystal and powder diffraction, as measured on a 2-dimensional detector (such as film):

Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.

An X-ray diffractometer

The photograph below shows a typical powder diffractometer

Photograph of labelled x-ray diffractometer

The X-ray beam comes from the tube, though slits, is diffracted from the sample, goes though another set of slits, diffracted from the secondary beam monochromator and measured by the detector.

The video below shows how the sample moves though θ (~5 to 45 ° ) while the detector scans though 2θ (~10 to 90 ° ). It has been speeded up as typical data collection time would be somewhere between 10 mins and 10 hours.

Video of a Diffractometer and the θ-2θ arrangement

The simulation below shows how the powder diffraction pattern of a simple face-centred cubic structure is influenced by changes in the cell parameter, atomic number, crystallite size and what happens when the material becomes amorphous.

Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.


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