Diffraction

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PANEL_06.inddWe have explained that crystals are solids in which matter is periodically ordered. This idea was developed scientifically in the nineteenth century. But its experimental confirmation was achieved in a crucial experiment that took place in 1912: the demonstration of the diffraction of X-rays by crystals. It was one of the most important and elegant experiments in the history of science.

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In 1895, Wilhelm Röntgen had discovered a radiation of unknown nature, which is why he called it X. It was capable of passing through solid bodies, but was it a wave or particles?

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Wilhelm Röntgen The famous image of his wife’s left hand

A century before it had been proposed that crystals were formed by tiny pieces of matter, the integrant molecules, periodically ordered in the space formed by a three-dimensional network. If crystals were really made by periodic stacking, and if X radiation was wavelike in nature, then crystals should function like a three-dimensional lattice that would diffract the X-rays. This brilliant experiment was designed by Max von Laüe and carried out by Paul Knipping and Walter Friedrich.

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Max von Laüe The first X-ray diffraction photograph

The experiment worked. It happened because the distance between the atoms in a crystal was similar to the wavelength of X-rays. The first successful diffraction photograph showed some sporadic, blurred but clearly discrete stains. More experiments followed and Laue demonstrated that the intensity and position of the stains of the diffraction photograph were related to the atomic structure of the crystal.

That same year, William Bragg and his son Lawrence tackled the problem in a different way. They conjectured that each plane of the crystal reflected the X radiation. When the difference in the distance travelled by the incidental radiation after being reflected in each plane of the crystal was a multiple of the L wavelength, the interference of the beams was constructive and in the radiography a stain appeared. In other words, when what is known as Bragg’s Law is fulfilled:

2 d sen θ = nλ

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The Braggs determined the structure of sodium chloride, table salt. They demonstrated that the structure was formed by a series of planes, each plane of the structure being like a chessboard in which the chlorine occupied the white squares and the sodium the black. The three-dimensional structure was obtained stacking chessboards so that the white squares of the upper chessboard were positioned on top of the black squares of the board below. Each chlorine atom was thus surrounded by six sodium atoms, and viceversa.

image021 sodium chloride crystal structue

It was fantastic. The diffraction pattern depended on the distance between planes and the type of atoms that made it up. That is to say, it was unique. Like our fingerprints, this pattern was the identity card for minerals and chemical compounds. We finally had an unequivocal technique for identifying and classifying matter.

But there was more. It was possible not only to identify matter but also to understand its structure, to know how it was organised at the atomic level, which would open the doors not only to understanding matter but also to manipulate it physically and chemically.

Common salt was followed by other structures, also simple, such as diamond, copper and pyrite. But to succeed in revealing molecular structures with a greater number of atoms was an almost impossible undertaking.

Nearly all the scientists who appear in this panel received the Nobel Prize: Röntgen in 1901 for the discovery of X-rays; Laue in 1914 for demonstrating crystal X-ray diffraction; and William Lawrence Bragg and his father William Henry Bragg in 1915 for their studies elucidating the structure of crystalline matter by X-ray diffraction.

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