Single Crystal versus Powder Specimens

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Single Crystal versus Powder Specimens

As already remarked, the original discovery of diffraction was made using a single crystal rather than a powder-specimen (see first page: In the Beginning). Single crystal diffraction has dominated crystallography ever since though powder diffraction has had its moments and has seen a sustained revival over the last 25 years.

The logic of using single crystals has already been outlined in the previous page: in short, a single crystal provides an unambiguous way in which a d_hkl can be chosen (fixed) and rotated (theta scanned) to yield a diffraction event. However powder specimens provide for an alternative strategy in which crystal rotation is not necessary: i.e. the (literally) millions of crystallites present in a powder specimen are randomly oriented; there is no requirement for rotation since, statistically, all possible orientations are represented in the powder i.e. those crystallites at the appropriate orientation satisfying Bragg's Law will diffract the X-rays, whereas those at the wrong orientation do not diffract. The simplicity arising from not having to grow a sufficiently large crystal nor having to provide an arranged sample rotation can be considerable in certain cases though this gain, we will see later, is achieved at some cost.

We need to consider what are the ideal characteristics of a powder specimen. First the individual crystallites composing the specimen must be highly crystalline; the crystallites must be randomly oriented to represent all possible crystal orientations; the size of the crystallites must be small enough such that a sufficiently large statistical number can be present within a powder specimen to represent all possible orientations; however, the size of the crystallites must also be large enough such that poor diffraction does not result from certain adverse effects (which will be dealt with later) of too small a crystallite size.

If the crystallites have highly asymmetric shapes (e.g. plates or needles) they will tend to become aligned when packed together as a powder specimen; the best shapes are termed equi-axed, the closest realistic approximation to the ideal shape which would be a sphere. It is found empirically that, for most materials, a crystallite size of between 1 and 10 µm (1 µm = 10 ^-3 mm = 10 ^-6 m) is ideal. For example, an ideal crystallite with approximately cubic shape might have a volume of 1 µm³ (i.e. 10 ^-9 mm³ or 10 ^-36 m³ ).

The rest of this course will concentrate on powder specimens, rather than single crystals, in the use of diffraction.

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