Editing Neutron diffraction (section)

==Uses==
Neutron diffraction can be used to determine the [[static structure factor]] of [[gas]]es, [[liquid]]s or [[amorphous solid]]s. Most experiments, however, aim at the structure of crystalline solids, making neutron diffraction an important tool of [[crystallography]].

Neutron diffraction is closely related to X-ray [[powder diffraction]].<ref>''Neutron powder diffraction'' by Richard M. Ibberson and William I.F. David, Chapter 5 of Structure determination form powder diffraction data IUCr monographphs on crystallography, Oxford scientific publications 2002, {{ISBN|0-19-850091-2}}</ref> In fact, the single crystal version of the technique is less commonly used because currently available neutron sources require relatively large samples and large single crystals are hard or impossible to come by for most materials. Future developments, however, may well change this picture. Because the data is typically a 1D powder diffractogram they are usually processed using [[Rietveld refinement]]. In fact the latter found its origin in neutron diffraction (at Petten in the Netherlands) and was later extended for use in X-ray diffraction.

One practical application of elastic neutron scattering/diffraction is that the [[lattice constant]] of [[metal]]s and other crystalline materials can be very accurately measured. Together with an accurately aligned micropositioner a map of the lattice constant through the metal can be derived. This can easily be converted to the [[Stress (physics)|stress]] field experienced by the material.<ref name="iaea" /> This has been used to analyse stresses in [[aerospace]] and [[automotive]] components to give just two examples. The high penetration depth permits measuring residual stresses in bulk components as crankshafts, pistons, rails, gears. This technique has led to the development of dedicated stress diffractometers, such as the [[ENGIN-X]] instrument at the [[ISIS neutron source]].

Neutron diffraction can also be employed to give insight into the 3D structure any material that diffracts.<ref name="ojeda">{{citation|author= Ojeda-May, P.|display-authors= 4|author2= Terrones, M.|author3= Terrones, H.|author4= Hoffman, D.|author5= Proffen, T.|author6= Cheetham, A. |title=Determination of chiralities of single-walled carbon nanotubes by neutron powder diffraction technique|journal=Diamond and Related Materials|date=2007|volume=16|issue= 3|pages=473–476|bibcode = 2007DRM....16..473O |doi = 10.1016/j.diamond.2006.09.019 }}</ref><ref name="page">{{citation|author= Page, K.|author2= Proffen, T.|author3= Niederberger, M.|author4= Seshadri, R. |title=Probing Local Dipoles and Ligand Structure in BaTiO3 Nanoparticles|journal=Chemistry of Materials|date=2010|volume=22|issue= 15|pages=4386–4391|doi=10.1021/cm100440p}}</ref>

Another use is for the determination of the [[solvation shell|solvation number]] of ion pairs in electrolytes solutions.

The magnetic scattering effect has been used since the establishment of the neutron diffraction technique to quantify magnetic moments in materials, and study the magnetic dipole orientation and structure.  One of the earliest applications of neutron diffraction was in the study of magnetic dipole orientations in [[Antiferromagnetism|antiferromagnetic]] transition metal oxides such as manganese, iron, nickel, and cobalt oxides.  These experiments, first performed by Clifford Shull, were the first to show the existence of the antiferromagnetic arrangement of magnetic dipoles in a material structure.<ref>{{cite journal | last1=Shull | first1=C. G. | last2=Strauser | first2=W. A. | last3=Wollan | first3=E. O. | title=Neutron Diffraction by Paramagnetic and Antiferromagnetic Substances | journal=Physical Review | publisher=American Physical Society (APS) | volume=83 | issue=2 | date=1951-07-15 | issn=0031-899X | doi=10.1103/physrev.83.333 | pages=333–345| bibcode=1951PhRv...83..333S }}</ref> Now, neutron diffraction continues to be used to characterize newly developed magnetic materials.

===Hydrogen, null-scattering and contrast variation===
Neutron diffraction can be used to establish the structure of low atomic number materials like proteins and surfactants much more easily with lower flux than at a synchrotron radiation source. This is because some low atomic number materials have a higher cross section for neutron interaction than higher atomic weight materials.

One major advantage of neutron diffraction over X-ray diffraction is that the latter is rather insensitive to the presence of [[hydrogen]] (H) in a structure, whereas the nuclei <sup>1</sup>H and <sup>2</sup>H (i.e. [[Deuterium]], D) are strong scatterers for neutrons. The greater scattering power of protons and deuterons means that the position of hydrogen in a crystal and its thermal motions can be determined with greater precision by neutron diffraction.  The structures of [[metal hydride complex]]es, e.g., [[Magnesium iron hexahydride|Mg<sub>2</sub>FeH<sub>6</sub>]] have been assessed by neutron diffraction.<ref>Robert Bau, Mary H. Drabnis "Structures of transition metal hydrides determined by neutron diffraction" Inorganica Chimica Acta 1997, vol. 259, pp/ 27-50. {{doi|10.1016/S0020-1693(97)89125-6}}</ref>

The neutron scattering lengths ''b''<sub>H</sub> = −3.7406(11) fm <ref name="Sears">{{citation|author=Sears, V. F.|title=Neutron scattering lengths and cross sections|journal=Neutron News|date=1992|volume=3|issue=3|pages=26–37|doi=10.1080/10448639208218770}}</ref> and ''b''<sub>D</sub> = 6.671(4) fm,<ref name="Sears" /> for H and D respectively, have opposite sign, which allows the technique to distinguish them.  In fact there is a particular [[isotope]] ratio for which the contribution of the element would cancel, this is called null-scattering.

It is undesirable to work with the relatively high concentration of H in a sample. The scattering intensity by H-nuclei has a large inelastic component, which  creates a large continuous background that is more or less independent of scattering angle. The elastic pattern typically consists of sharp [[Bragg reflections]] if the sample is crystalline. They tend to drown in the inelastic background. This is even more serious when the technique is used for the study of liquid structure. Nevertheless, by preparing samples with different isotope ratios, it is possible to vary the scattering contrast enough to highlight one element in an otherwise complicated structure. The variation of other elements is possible but usually rather expensive. Hydrogen is inexpensive and particularly interesting, because it plays an exceptionally large role in biochemical structures and is difficult to study structurally in other ways.