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Celebrating 100 years of X-ray crystallography

2012; Wiley; Volume: 69; Issue: 1 Linguagem: Inglês

10.1107/s0108767312048490

1600-5724

S. Wilkins,

Crystallography and Radiation Phenomena

marks the centenary of the reading of the paper by William Lawrence Bragg (WLB) to the Cambridge Philosophical Society outlining the foundations of X-ray crystallography.It included the derivation of the first correct atomic structure of a crystal, namely that of zinc blende, based on the X-ray diffraction pattern recorded by Friedrich, Knipping and Laue in the spring of 1912.At the time of the lecture, Lawrence Bragg (as he later preferred to be called) was 22 years old and still a research student in the Cavendish Laboratory.This special issue of Acta Crystallographica Section A is dedicated to commemorating this landmark scientific event, the ramifications of which have served as a wellspring for many other branches of science, including structural chemistry, mineralogy, materials science, solid-state physics and molecular biology.It is very fitting that a Bragg centennial issue be published in Acta Crystallographica, as it was Lawrence Bragg who was a major influence behind the formation of the International Union of Crystallography (IUCr) and Acta Crystallographica more than six decades ago.Lawrence Bragg also served as the inaugural President of the IUCr Executive Committee.A sense of the development of crystallography over the past six decades can be obtained by looking at the most frequently cited articles in Acta Crystallographica as listed on the IUCr Journals website.The breakthrough by Lawrence Bragg did not come in isolation (e.g.see Ewald, 1962; Phillips, 1979, p. 88;Perutz, 1990;Jenkin, 2008Jenkin, , 2012)).Having entered the Cavendish to work under J. J. Thomson, he lost interest in the project he was given and in the summer of 1912 he joined the family for a vacation on the Yorkshire coast.There his father, William Henry Bragg (WHB), then Professor of Physics at Leeds, was pondering on a letter from Germany reporting on the experiments of Laue and colleagues, and discussed this work with Lawrence.While strongly suggesting that X-rays were waves and diffracted by the crystal, the explanation by Laue appeared incomplete and unconvincing.William had for some time held to a neutral-pair hypothesis for the nature of X-rays.Not yet convinced of the wave nature of X-rays by the Laue experiments, William continued to adhere to his neutral-pair model, especially in lively exchanges with Barkla, although he was groping for a reconciliation between apparent particle and wave-like properties of X-rays.On returning to Cambridge, Lawrence reflected more deeply on the nature of the pattern recorded by Laue and colleagues.Some key issues were that Laue had assumed that the spectrum from his X-ray source was so impure that what they needed to do to get observable diffraction effects was to use their X-ray source to generate fluorescence in the crystal.It would be, they assumed, the fluorescent radiation that would be diffracted by the same crystal (i.e. they were seeking to observe what subsequently became known as the X-ray Kossel effect).For this reason, they chose copper sulfate as one of their early samples for study in order to excite fluorescence by the relatively heavy atoms of copper.Lawrence grappled with and sought to treat correctly three basic aspects of the phenomenon observed by Laue and colleagues, namely:(i) the nature of the radiation spectrum being diffracted by the crystal;