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What is a Powder?

Perhaps we should first ask, what is a crystal? A crystal is built up by the adding, in three dimensions, of identical building blocks which crystallographers call a unit cell. For example below we see (unreal) "theoretical" crystals of copper sulphate hydrate, as built on a computer-graphic system, consisting of 8, 27, 64 and 125 unit cells.

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Of course real crystals have much larger numbers of unit cells. In every day life we are used to seeing crystals (e.g. diamond, quartz, silicon) with our naked eye, and these might typically consist of 1018 unit cells.

What is not so apparent is that many materials we call powders (e.g. talcum powder, cement powder) are actually comprised of very small crystals which are known as "crystallites". If we view these powders under an electron microscope we can easily distinguish the crystallites: sometimes they appear just like minute crystals, that is scaled down versions of larger crystals; other times, as in the example below, they may have no obvious distinguishing morphology and it only becomes apparent that they are crystals by the fact that they produce a diffraction pattern. In the example below, a scanning electron micrograph of zirconia is shown together with its X-ray powder diffraction pattern: the latter is shown as a plot of intensity versus angle and will be explained more fully later. Suffice to say at present that whereas diffraction occurs in "all directions" a powder diffraction pattern effectively compresses this information into one dimension, the 2θ-dimension.

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It seems both cruel and kind that crystallite powders should be so ubiquitous in real life: Cruel in that many important materials (particularly drugs, catalysts, super-conductors) usually do not form large crystals for study; but also kind in that at least by forming crystallites there is an alternative avenue of study, powder diffraction. It is also important to note that many materials that do not appear to be powdered in the normal sense of the word, do in fact behave as powders from a diffraction point of view: drugs (where the powder has been compacted into a tablet) and metals (where the crystallites are held together by grain boundaries) are two common examples.

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© Copyright 1997-2006.  Birkbeck College, University of London.
Author(s): Paul Barnes
Tony Csoka
Martin Vickers