Neutron diffraction is an elastic scattering technique where the neutron does not exchange energy with the sample material during the scattering process.  Beams of thermal or cold neutrons (which travel at velocities in the approximate range 500-5000m/s) have associated wavelengths comparable to interatomic distances and so will produce diffraction patterns when they scatter from crystalline materials in a process similar to that observed with X-rays, thereby allowing the positions of atoms within the unit cells to be determined. 

Nuclear Scattering 

However in contrast to X-ray diffraction, where the X-rays are scattered by electrons, neutrons are scattered by atomic nuclei.  As a result the scattering power of an atom for neutrons does not increase linearly with atomic number but instead varies in an almost random manner across the periodic table.  This has a number of important consequences and means neutron diffraction can provide information complementary to that obtained using X-rays:

• elements close to one another in the periodic table can have very different scattering lengths, for example it is easy to distinguish between transition metals using neutrons.
• isotopes of the same element may have different scattering lengths (e.g. hydrogen and deuterium) allowing diffraction experiments to utilise isotopic labelling.
• light elements can make a significant contribution to the overall scattering in the presence of heavy ones. e.g. Oxygen has a scattering power similar to many of the Rare Earths;  and the scattering  power of Lithium (atomic number 3) is 25% that of Uranium (atomic number 92).
• some elements have scattering lengths very close to zero which means that the intensities of their Bragg reflections are very small, e.g. vanadium is frequently used as a sample container because its scattering makes negligible contribution to the diffraction pattern. 

Furthermore, because the interaction with the nucleus occurs over very short distances there is no form factor in a neutron diffraction experiment allowing diffraction data to be collected to large values of (sinθ)/λ. In addition, time-of-flight experiments at pulsed neutron sources allow the collection of powder patterns with very high Δd/d​ resolution in reciprocal space. Combined, these two effects enable neutron diffraction to make very precise determinations of both average and local structures in materials. 

Finally, because neutrons are uncharged their interaction with matter is weaker than that of X-rays, meaning that neutron diffraction probes bulk structure and is influenced less by near-surface effects. This also means that neutrons pentrate deep into (and out of) materials and enables experiments to be carried out on samples under non-ambient conditions in complex environments such as at extremes of pressure, temperature or magnetic field. 

Magnetic scattering 

Although neutrons are uncharged they carry a spin, meaning that as well as scattering from atomic nuclei the neutron can interact with electron spins and induced orbital moments.  This makes neutron diffraction an ideal technique to provide information on the location and orientation of the magnetic moments within a magnetic material. 

Because the magnetic interaction occurs over distances comparable to neutron wavelengths, magnetic neutron scattering is attenuated by a form factor in a similar manner to the X ray atomic form factor.  However, the magnetic form factor is much more pronounced and decreases much more rapidly as a function of (sinθ)/λ and as a result the magnetic Bragg reflections are strongest for long d-spacings that are typically measured in detector banks at low 2θ angles.