Celebrating crystallography

In this section, we invite our readers to tell us about their work, lives and scientific passions, Here, Bob Newport, Professor of Materials Physics at the University of Kent, tells us about the year of crystallography This is the International Year of Crystallography. There’s an overview of the century-old field in a recent special edition of Nature and in several blogs. In the UK, which has played a leading role throughout, the Royal Institution’s website adds an excellent suite of accessible articles – including a fascinating timeline. However, I want to celebrate materials which are not crystals: the study of which extends back almost as far and uses many of the same empirical tools. It is of the essence of a crystal that its atoms are arranged in a very specific way which extends throughout its volume. Thus, if we know the positions of a suitable sub-set of them then we can predict the positions of all the others: we can reliably extrapolate from the microscopic to the macroscopic. For much of the past century, X-ray diffraction was the initial experimental step in this process. The advent of synchrotron X-ray sources like the Diamond facility, allowed much of modern crystallography to flourish. Exploiting the wave-like properties of the neutron, using research facilities such as ISIS and the ILL, adds a new dimension because neutrons scatter from nuclei whereas X-rays interact with an atom’s cloud of electrons – so, we have access to the lighter elements for example. These developments have also opened up the study of materials, such as glasses, which are not crystalline. Silicate glasses are everywhere – from plate glass windows to the surface of the moon to bioactive scaffolds for the regeneration of bone – and based on a deceptively simple 3D network of silicon and oxygen atoms. Driven by the basic rules of chemical bonding, each silicon atom is bonded to four oxygens to make a tetrahedral unit; two of those oxygens are themselves part of neighbouring tetrahedra, but there is a small variation in the angles between each of these units. That’s all it takes to ensure that the short-ranged tetrahedral ordering cannot generate the long-range order needed to form a crystal. Despite this, many of the same tools deployed to understand a crystal may be used to study a glass or other amorphous material. The first X-ray diffraction experiments on a glass were undertaken in the 1920-30s, but its modern X-ray or neutrons methods that provide detailed, albeit statistically averaged, insights into the arrangement of its atoms.