X-ray crystallography

… the properties of the metals must depend, in the first place, on the properties of the individual atoms, and in the second place, on the atomic arrangement which is in effect the state of crystallisation. In the very centres of the metal industries it has been realised of recent years that the scientific observer with his microscope can bring some system into the mass of disordered knowledge, and can improve the quality of the manufacture and the certainty of its production. Yet … the microscope can only go to a certain length: it stops far short of the point we must reach if we are to understand how the atoms are acting so as to give the various materials their specific properties. It can show the existence of separate crystals in the metal, but not the arrangement of the atoms in the crystals … in the X-rays, we find a new hope …
— William Henry Bragg (1862 – 1942), ‘The Nature of Crystals: Metals’ in Concerning the nature of things.

[My purpose] is to go back to the beginning and broadly summarize the course of X-ray crystallography over the past half-century or so. In so doing, I shall try to answer two key questions: Why X-rays? Why crystals?

X-ray crystallography is a strange branch of science. The results of investigations lasting many years can be summed up in a ‘model’. I have often been asked: ‘Why are you always showing and talking about models? Other kinds of scientists do not do this.’ The answer is that what the investigator has been seeking all along is simply a structural plan, a map if you will, that shows all the atoms in their relative positions in space. No other branch of science is so completely geographical; a list of spatial coordinates is all that is needed to tell the world what has been discovered …

This brings us to the question of why X-rays, of all the available forms of electromagnetic radiation, are indispensable for this method of investigation. In order for the interference of the diffracted beams to produce marked changes in the amount of scattering in different directions, the differences in the paths taken by reflected beams must be on the order of a wavelength. Only X-rays have wavelengths short enough to satisfy this condition. For example, the distance between neighbouring sodium and chlorine atoms in a crystal of sodium chloride (ordinary table salt) is 2.81 angstrom units … whereas the most commonly used wavelength in X-ray analysis is 1.54 Angstroms.
— William Lawrence Bragg (1890 – 1971), ‘X-ray crystallography’, Scientific American, July 1968.

[The account below has been lightly edited, in order to remove references to other chapters in von Laue’s book.]

At first these studies had no effect on physics because no physical phenomenon required the acceptance of the space lattice hypothesis. Among the few physicists who were at all interested in crystallography, some adopted the opposite view, that in crystals, as elsewhere in matter, the molecular centres of gravity were distributed irregularly and that only the parallel placing of preferred directions in the molecules produced anisotropy. Neither was there much discussion of the hypothesis in mineralogy. Paul von Groth [1843 – 1927] alone upheld the Sohncke tradition in his teaching in Munich. The triumph of this hypothesis came in 1912 through the experiments of W. Friedrich and Paul Knipping who, by means of X rays, demonstrated the interference phenomena occasioned by the space lattice of the crystal, a finding which verified the prediction of M. von Laue. Because of their short wavelength, these waves are able to reveal optically the interatomic distances, whereas these elude radiations of longer wavelengths, such as light. These experiments also furnished the first decisive proof of the wave nature of X rays, which up to then had been denied by some eminent scientists because of the particularly striking quanta phenomena shown by them. The theory of this interference phenomenon, which Laue suggested in his first paper, and which was verified quantitatively, is an easy generalisation of the theory given by Schwerd in 1835 for optical gratings. The finding was doubted by some but not for long because the few sharp interference maxima of X rays are too suggestive of optical grating spectra. Though only an approximation, time has proved the theory to be an astoundingly close approximation. Here the wave theory of X rays and the atomic theory of crystals come together, one of these surprising events to which physics owes its powers of conviction.
— Max von Laue (1879 – 1960), History of Physics, trans. Ralph Oesper, Academic Press, 1950, 120 – 1.

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