Friday, 7 February 2014

Celebrating Disorder

The following is a much extended version of the guest blog I recently wrote for Laboratory News UK.

This is the International Year of Crystallography. For those of us with an interest in the science of materials, it’s been hard to miss. There have been innumerable events and projects to mark a century of theoretical and experimental work aimed at understanding the arrangement of atoms in crystals (i.e. their atomic-scale structure). Some of this expanse of science has gone beyond famous and has become ‘everyday’ – like the unravelling of the structure of DNA; there’s an overview 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 web site adds an excellent suite of accessible articles – including a fascinating timeline.

However, I want to celebrate the structure of materials which are not crystals: the study of which extends back almost as far and uses many of the same empirical and computational tools.

It is of the essence of a crystal – whether that be of common table salt, NaCl, or of a fiendishly complex biological molecule – that its constituent atoms are arranged in a very specific order. If we know the positions of a suitable sub-set of them then we can predict the positions of all the others: we use the concepts of unit cells, lattices and symmetry if order to extrapolate from the microscopic to the macroscopic. For much of the past century, X-ray diffraction was the initial experimental step in this process. Whilst laboratory-based X-ray sources continue to be used to great effect, it’s arguably the case that the advent of the synchrotron source, such as the UK’s Diamond facility, allowed much of modern crystallography to expand. Exploiting the wave-like properties of the neutron, using research facilities such as ISIS and the ILL, adds a new dimension. Why? Because neutrons interact with nuclei whereas X-rays scatter from an atom’s cloud of electrons – thus, we have access to the lighter elements (i.e. those which scatter X-rays relatively little) and we can use the fact that one of a given element’s isotopes may scatter neutrons very differently to another. Add to this the fact that a neutron has mass, unlike an X-ray photon, and one might now even look at the movement of atoms in a material.

These developments have opened up the study of materials which are not crystalline, where the basic rules of chemical bonding still hold true but the long-range ordering of the crystal is absent. A glass is one example of an amorphous, or non-crystalline, material. Silicate glasses are everywhere – from plate glass windows to the surface of the moon, from the skeletons of deep sea sponges 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 our short-ranged tetrahedral ordering cannot generate the long-range order needed to form a crystal. On the left hand side of the image, I’m holding a crystal of quartz (crystalline SiO2) and in the other hand I have a silicate glass (chemically similar, although with some metal ions present, giving the colour). Both these solids have almost identical local atomic arrangements, but only one of them is a crystal. 

This brings us to the link with the ‘International Year of Crystallography’. Despite the lack of ordering, many of the same tools deployed to understand a crystal may be used to study a glass, or any other amorphous material. Thus, diffraction methods using X-rays or neutrons provide detailed, albeit statistically averaged, insights into the arrangement of atoms in our glassy materials. Indeed, given the structural, and often chemical complexity of a glass it becomes all-but essential to use both X-rays and neutrons (including playing games with the mix of isotopes) and a whole bunch of other physical probes if one is to understand them fully. In fact, the first X-ray diffraction experiments on a glass were undertaken in the 1920s, with analysis methods becoming quite refined within a decade [1]. However, as with crystallography, it has been the advent and development of modern neutron and synchrotron X-ray facilities that has provided the tools for a sophisticated, quantitative understanding of the relationship between atomic-scale structure and observed properties.

One of these days I must write something longer on the science, art and technology of glass– not today though … . In the meantime, take a look at some of the videos on the subject available via YouTube: one recent favourite of mine, on why window glass is transparent, was put together by Prof. Mark Miodownik, who is a well known and highly accomplished science communicator in the materials arena. I have also uploaded a recording of one of my own talks to YouTube which was delivered a few years back at Canterbury’s Heritage Museum; progressively modified/updated versions of this talk have been delivered several times since then – with more bookings already agreed.

Most, if not all my published output on the structures of amorphous materials (including a lot on glasses and glassy materials) are available via ResearcherID or Google Scholar (search using "Robert Newport"); you can also find me on ResearchGate. New papers are touted via Twitter as well: @Bob_MatPhys.

[1] It is of passing interest that, at the same time as the Bragg father and son team were conducting their first X-ray diffraction experiments – i.e. a century ago – the famous glass technologist Otto Schott was suppressing a book on glass (The Chemical Technology of Glass, Eberhard Zschimmer ,1913) written within his factory (Jenaer Glaswerke Schott und Genossen) because he was concerned that some of his commercial secrets might leak out. The book has recently been translated into English and is available from the Society of Glass Technology, from whose news feed this snippet is derived.

No comments:

Post a Comment