Adventures in reciprocal space - from Laue to Bragg and back again

Abstract

The earliest X-ray diffraction experiments [1] employed radiation “as generated” without monochromation to produce the reciprocal space images we know as Laue patterns. The pioneering work of W.L. Bragg [2] using monochromated X-rays followed rapidly and provided the major simplification in the mathematics required to analyse X-ray diffraction patterns to derive data from which atomic resolution structural information can be deduced. In the following century, physicists, chemists and later biologists developed the Bragg methodology into a powerful tool which underpins the structure based paradigm at the core of modern chemistry and biology. Application of the Laue method to questions of structure determination at atomic resolution languished for many decades until the availability of fast computers and the technical challenges of Synchrotron sources led to a resurgence in this experimental approach [3]. With the cost of neutron beams being substantially more than that of X-ray beams, the applicability of this method to neutron diffraction studies was soon investigated [4] and today Laue neutron diffraction is the method of choice for the determination of structures where neutron diffraction is scientifically required to prove aspects of structure for which X-ray diffraction can only be “suggestive”. Chemists typically employ an array of physical methods to support their structural assertions, but a crystal structure is often presented as absolute proof and as justification of inferior characterization by other methods. This being the case it is of great concern that crystallographic studies be critically reviewed by both analyst and in the publication process. Checkcif is a fine tool but insufficient to ensure the integrity of the scientific literature – that is properly the role of the analyst and the reviewers.