Standards

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Standards

Before expending energy and time in attempting to index a powder diffraction pattern, it is essential to know that the data are reliable and are not plagued by systematic and other unknown errors. Data reliability can be considerably enhanced by pre-checking the diffractometer with known standard samples. It is good laboratory practice to do this always when using an instrument aligned, configured, set-up, etc., by someone else; for example, when carrying out experiments external to the home laboratory, say, at synchrotron or neutron facilities. Even in the home laboratory, it is advisable to run standard samples on a regular basis to check that instrument performance has not degraded over a period of time. A standard sample can usually be measured relatively quickly and will provide information on instrument calibration, alignment, resolution, background count, source flux, spurious scattering (if any) from sample environment equipment, and so on. You should never be put off from doing this by so-called "local experts" (especially at synchrotron and neutron facilities) who may insist that their instrument is well-characterised and perfectly configured: experts are not infallible!

So what type of material makes a good calibrant for a powder diffractometer? The material should be of high symmetry because the intensity of the diffraction planes is concentrated into relatively few diffraction peaks (see later discussion on multiplicity). The unit-cell volume, V, should be small since the intensity of the diffraction peaks, I(hkl), is inversely proportional to V. Ideally, the unit cell should contain only 1 or perhaps 2 crystallographic atoms with large scattering factors f or b (for X-rays and neutrons, respectively). The thermal vibrations of the atom (or atoms), characterized by its B value, should be as small as possible so that the high-angle peaks have maximum intensity. It is useful if the sample absorption is not too high since this can affect, in extreme cases, the position of the powder lines in addition to reducing their intensity. It must also be possible to obtain large quantities of the material in high purity and crystallinity together with reproducible crystallite size. Obviously, the materials must be air stable and preferably non-toxic.

Currently, the National Institute of Standards and Technology (NIST), formerly known as the National Bureau of Standards (NBS), at Gaithersburg, Washington D.C., U.S.A., supplies most of the world's standard calibrating materials (in return for a suitable sum of money!). Typical standards are powdered Si, Ni, ZnO, TiO₂, CeO₂, Al₂O₃, Cr₂O₃, and Y₂O₃. These samples can be used as calibrants for both X-ray and neutron powder diffraction. Note that a simple material such as NaCl does not make a good standard because it is hygroscopic and the Na⁺ and Cl^- ions have large thermal parameters due to their single charge. All the materials listed above as standards have rigid-lattice structures due to either metallic bonding, or covalent bonding, or ionic bonding with highly charged cations and anions.

It is important to realize that some standards may be excellent for one purpose (e.g. wavelength calibration) and less useful for another (e.g. determination of instrumental resolution), so choose a standard appropriate for the task in-hand. Note that most common use of standard samples is for wavelength calibration, but that the same sample can also be used to check for spurious scatter so long as the complete diffraction pattern is measured and not just the profiles of the known Bragg reflections.

The standards quoted above all have small unit cells and hence small d spacings. Through the Bragg equation, this implies that they do not give diffraction peaks at low scattering angles. The best samples to check the performance of the diffractometer at low angles are layer-like: mica is one such material supplied by NIST, but a more useful one is silver behenate, which has a layer spacing of 58.38 Å (reference [1] below and references therein).

[1] T.N.Blanton, C.L.Barnes, M.Lelental. J. Applied Cryst. 2000, 33, 172-173.

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