Diffraction

Why is structure important?

The influence of structure is everywhere; the properties of water and ice, the hardness of metals, the strength of magnets, and even the biology of DNA or the effect of antibodies on viruses – all depend on structure. For example, the structure of gold consists of close-packed atoms, much like a stack of oranges in the local supermarket. The planes of oranges can easily slip over each other, and for largely similar reasons gold is readily malleable.

There is another slightly less close-packed structure in which atoms are arranged at the corners of a cube, with another atom at the centre. The planes of atoms in this structure can slip less easily, and metals that adopt it, such as chromium, are less malleable.

This is a trivial example, and there are other structural reasons why some metals are harder – to do with 'impurities' and imperfections, which also pin atomic planes and stop them from slipping.

The articles published in the "chemistry" section of the ILL Annual Report show many examples of the relation between the structure and properties of new materials.

We generally use a large crystal monochromator to select a particular neutron wavelength, just as the different wavelengths of light can be separated using a prism or fine grating. The material to be studied is placed in this monochromatic neutron beam, and the scattered neutrons are collected on a large 2D detector. The sample can be a liquid, a bunch of fibres, a crystal or a polycrystal. A polycrystal is the usual form of solid matter, such as a lump of metal or ceramic, and is made up of millions of tiny crystals.

To understand how neutron diffraction works, imagine how light is diffracted by a regular grating or grid. Scattering from the different lines of the grid interferes to give diffraction "spots" with spacing inversely proportional to the spacing of the lines. X-ray and neutron diffraction work the same way, but the grid is now the array of atoms in the material. By measuring the intensities and positions of the scattered X-ray or neutron spots,we can deduce the atomic structure.

Neutron diffraction experiments at ILL are thus really quite simple, and available to a wide variety of users – materials scientists, chemists, physicists and biologists. The simplest is called "powder diffraction", when a polycrystalline lump of material, often ground to a fine powder, is placed in the beam. Neutrons are scattered at specific angles, corresponding to the spacing between atomic planes, and by measuring these angles and intensities the atomic structure of the material can be deduced. If instead of a crystalline powder an amorphous or liquid sample is used, there are only broad peaks at specific angles corresponding to average interatomic distances.

To obtain more data, short neutron wavelengths are used, and sometimes one type of atom is replaced by its isotope – chemically identical, but with a different nucleus and different neutron-scattering power – this difference then gives information specific to that atom.