Neutron diffraction

Key point: The scattering of neutrons by crystals yields diffraction data that give additional information on structure, particularly the positions of light atoms. Diffraction occurs from crystals for any particle with a velocity such that its associated wavelength (through the de Broglie relation, λ= h/mv) is comparable to the separations of the atoms or ions in the crystal. Neutrons and electrons travelling at suitable velocities have wavelengths of the order of 100–200 pm and thus undergo diffraction by crystalline inorganic compounds. Neutron beams of the appropriate wavelength are generated by ‘moderating’ (slowing down) neutrons generated in nuclear reactors or through a process known as spallation, in which neutrons are chipped off the nuclei of heavy elements by ac celerated beams of protons. The instrumentation used for collecting data and ana lysing single-crystal or powder neutron diffraction patterns is often similar to that used for X-ray diffraction. The scale is much larger, however, because neutron beam fluxes are much lower than laboratory X-ray sources. Furthermore whereas many chemistry laboratories have X-ray diffraction equipment for structure characterization, neutron diffraction can be undertaken only at a few specialist sources worldwide. The investigation of an inorganic compound with this technique is therefore much less routine and its application is essentially limited to systems where X-ray diffraction fails.

The advantages of neutron diffraction stem from the fact that neutrons are scattered by nuclei rather than by the surrounding electrons. As a result, neutrons are sensi tive to structural parameters that often complement those for X-rays. In particular, the scattering is not dominated by the heavy elements, which can be a problem with X-ray diffraction for most inorganic compounds. For example, locating the position of a light element such as H and Li in a material that also contains Pb can be impossible with X-ray diffraction, as almost all the electron density is associated with the Pb atoms. With neutrons, in contrast, the scattering from light atoms is often similar to that of heavy elements, so the light atoms contribute significantly to the intensities in the diffraction pattern. Thus neutron diffraction is frequently used in conjunction with X-ray diffraction techniques to define an inorganic structure more accurately in terms of atoms such as H, Li, and O when they are in the presence of heavier, electron rich metal atoms. Typical applications include studies of the complex metal oxides, such as the high-temperature superconductors (where accurate oxide ion positions are required in the presence of metals such as Ba and Tl) and systems where H atom positions are of interest. Another use for neutron diffraction is to distinguish nearly isoelectronic species. In X-ray scattering, pairs of neighbouring elements in a period of the periodic table, such as O and N or Cl and S, are nearly isoelectronic and scatter X-rays to about the same extent, therefore they are hard to tell apart in a crystal structure that contains them both. How ever, the atoms of these pairs do scatter neutrons to very different extents, N 50 per cent more strongly than O, and Cl about four times better than S, so the identification of the atoms is much easier than by X-ray diffraction.

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Absorption spectroscopy

Single-crystal X-ray diffraction

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