Editing Electron diffraction (section)

==== Polycrystalline pattern ====
{{anchor|Figure 12}}[[File:SpotToRingDiffraction.gif|thumb|Figure 12: Relation between spot and ring diffraction illustrated on 1 to 1000 grains of [[MgO]] using simulation engine of [[CrysTBox]]. Corresponding experimental patterns can be seen in '''Figure 13.''' |alt=A pattern showing how diffraction patterns from different grain build up to yield a ring pattern.]]

Diffraction patterns depend on whether the beam is diffracted by one [[single crystal]] or by a number of differently oriented crystallites, for instance in a polycrystalline material. If there are many contributing crystallites, the diffraction image is a superposition of individual crystal patterns, see [[#Figure 12|Figure 12]]. With a large number of grains this superposition yields diffraction spots of all possible reciprocal lattice vectors. This results in a pattern of [[concentric]] rings as shown in [[#Figure 12|Figure 12]] and [[#Figure 13|13]].<ref name="HirschEtAl" />{{Rp|location=Chpt 5-6}}

{{anchor|Figure 13}}{{multiple image
| align             = right
| width             = 150
| image1            = ringGUI input.png
| image2            = ringGUI quadrant.png
| footer            = Figure 13: Ring diffraction image of [[MgO]] as recorded (left) and processed with CrysTBox ringGUI (right, with indexing). Corresponding simulated pattern can be seen in '''Figure 12'''.
| alt1              = Experimental ring pattern from magnesium oxide.
| alt2              = A computer model of a ring diffraction pattern to go with the other image.
}}

Textured materials yield a non-uniform distribution of intensity around the ring, which can be used to discriminate between nanocrystalline and amorphous phases. However, diffraction often cannot differentiate between very small grain polycrystalline materials and truly random order amorphous.<ref>{{Cite journal |last1=Howie |first1=A. |last2=Krivanek |first2=O. L. |last3=Rudee |first3=M. L. |date=1973 |title=Interpretation of electron micrographs and diffraction patterns of amorphous materials |url=http://www.tandfonline.com/doi/abs/10.1080/14786437308228927 |journal=Philosophical Magazine |language=en |volume=27 |issue=1 |pages=235–255 |doi=10.1080/14786437308228927 |bibcode=1973PMag...27..235H |issn=0031-8086|url-access=subscription }}</ref> Here [[high-resolution transmission electron microscopy]]<ref>{{Cite journal |last=Howie |first=A. |date=1978 |title=High resolution electron microscopy of amorphous thin films |url=https://dx.doi.org/10.1016/0022-3093%2878%2990098-4 |journal=Journal of Non-Crystalline Solids |series=Proceedings of the Topical Conference on Atomic Scale Structure of Amorphous Solids |volume=31 |issue=1 |pages=41–55 |doi=10.1016/0022-3093(78)90098-4 |bibcode=1978JNCS...31...41H |issn=0022-3093|url-access=subscription }}</ref> and [[fluctuation electron microscopy]]<ref>{{Cite journal |last1=Gibson |first1=J. M. |last2=Treacy |first2=M. M. J. |date=1997 |title=Diminished Medium-Range Order Observed in Annealed Amorphous Germanium |url=https://link.aps.org/doi/10.1103/PhysRevLett.78.1074 |journal=Physical Review Letters |language=en |volume=78 |issue=6 |pages=1074–1077 |doi=10.1103/PhysRevLett.78.1074 |bibcode=1997PhRvL..78.1074G |issn=0031-9007|url-access=subscription }}</ref><ref>{{Cite journal |last1=Treacy |first1=M M J |last2=Gibson |first2=J M |last3=Fan |first3=L |last4=Paterson |first4=D J |last5=McNulty |first5=I |date=2005 |title=Fluctuation microscopy: a probe of medium range order |url=https://iopscience.iop.org/article/10.1088/0034-4885/68/12/R06 |journal=Reports on Progress in Physics |volume=68 |issue=12 |pages=2899–2944 |doi=10.1088/0034-4885/68/12/R06 |bibcode=2005RPPh...68.2899T |s2cid=16316238 |issn=0034-4885|url-access=subscription }}</ref> can be more powerful, although this is still a topic of continuing development.