This year marks the 100th anniversary of the discovery of X-ray diffraction and the 85th anniversary of electron diffraction (see Microscopy Pioneers). For most of the time since their introduction, microscopists have known these two techniques as the primary phase identification methods used in conjunction with various microscopies. However, these two diffraction methods also have played enormous roles in understanding the structure of matter, as well as the nature of both X rays and electrons.
For the 17 years after Roentgen discovered X rays in 1895, there was a bitter debate over whether X rays were particles or waves. In 1912 Max von Laue suggested that, if the wave hypothesis was correct and if the wavelengths of X rays were similar to the atomic spacing, perhaps periodic arrays of atoms could be used as diffraction gratings in a manner similar to light. He asked two assistants, Walter Friedrich and Paul Knipping, to aim a beam of X rays at a crystal to determine if diffraction occurs. Even though a triclinic crystal with the lowest symmetry was selected for the experiment, beams diffracted by the crystal were detected on film. This one experiment showed that X rays should be considered as waves.
But this first X-ray diffraction photograph showed no discernable pattern in the diffracted spots, a consequence of the low-symmetry of the crystal that was used. When the target crystal was changed to zinc blende, with known cubic symmetry, the pattern of spots on the film (afterwards called a Laue diffraction pattern) was highly symmetrical. Laue's 1912 publication of the results included a scheme for the interpretation of these spots, but William L. Bragg and his father William H. Bragg thought this to be rather complicated. In the summer of 1912 they began working through Laue's data to find a simpler explanation of the phenomenon. By applying the light optical diffraction equation to X-ray diffraction, WL developed his famous Bragg's Law: nλ = 2d sin θ. Shortly afterward, WL determined the first atomic structure of any substance, the face-centered cubic structure of sodium chloride.
These findings in 1912 were honored with Nobel prizes for Laue in 1914 and for the Bragg father and son team in 1915. From then on, X-ray diffraction permitted numerous other discoveries, the most notable of which was the determination of the double-helix structure of DNA by Watson, Crick, and Wilkins in 1953 (1962 Nobel prize in Physiology or Medicine).
The discovery of electron diffraction took place just a few years after de Broglie hypothesized in 1924 that fast atomic particles should have wavelike properties. In 1927 Davisson and Germer successfully obtained evidence for the diffraction of electrons from a crystal. This experiment proved that electrons had wavelike properties, confirming the wave-particle duality, and was the precursor for another new method for understanding the structure of matter. The discoveries and awards associated with the early work on electron diffraction are summarized in Wil Bigelow's article in this issue's Microscopy Pioneers section.
