What is diffraction?
X-ray diffraction is the best tool for determining atomic and molecular structure of crystalline matter. The electrons in the regular lattice of the crystal cause the incident X-rays to diffract into many specific directions. By measuring the angle of diffraction and the intensity of these diffracted beams it is possible to produce a three-dimensional map of the electron density. From this map, it is possible to determine the positions of the atoms in the crystal.
The technique can be used to study the atomic structure of single crystals, crystalline powders as well as nanoparticles and thin films.
X-rays interact with electrons and the contrast between elements is determined by the number of electrons in the atoms – i.e. a very heavy atom such as Uranium can completely hide an adjacent light atom such as Oxygen. In this case, it could be beneficial to use neutrons instead of X-rays since the contrast between elements in neutron scattering is widely different than X-rays. The same is true for e.g. materials containing neighboring transition metals where the contrast difference using X-rays is minimal but often much higher using neutrons.
A single crystal scatters X-rays (or neutrons) in discrete directions. The intensity of these diffracted beams are used to solve the atomic structure.
A powder consisting of a huge number of small crystals will give rise to concentric cones of scattered radiation. Often the data are reduced to a plot of intensity as a function of scattering angle, which can be used for identification and/or structural refinement.