Neutron Scattering as a useful tool to study clay minerals

Article

CDT Student Aasim Shaffi recently published a article in The Clay Minerals Group 75th Anniversary Jubilee Newsletter.

A copy of his article can be read below:

Neutrons go to the depths of clay

Neutrons are a non-destructive probe to study the bulk structure of clay minerals. Unlike X-rays, neutrons are particularly sensitive to hydrogen features in clays, such as hydroxyl groups and water, since hydrogen is a strong scatterer of neutrons. As part of my PhD studies, I’m studying interlayer water in halloysite, a 1:1 clay mineral classified in the kaolin group. Neutron experiments conducted at the ISIS Neutron and Muon Source (UK), will help to unravel the water structure and its dynamics. This knowledge could explain why halloysite tends to adopt a nanotubular form, which is the biggest driver of research in the field, due to its wide use in nanotechnology.

Neutron diffraction can give us structural information on varying length scales (Fig.1). For example, studies on swelling clays have tracked interlayer expansion on the angstrom scale (~1Å), to changes in the larger clay morphology on the sub-micron scale (~100nm). The latter is studied by small-angle neutron scattering (SANS). Neutron spectroscopy can give us dynamical information, extracted from the gain or loss of energy, as neutrons scatter from clay minerals. Small energy changes can identify diffusive and localised motion, on pico- and nanosecond time scales. Such experiments, termed quasi-elastic neutron scattering (QENS), include extracting diffusion coefficients of water in clays. This gives molecular insight to a process that is important in geology, agriculture, and engineering, such as the design of nuclear waste repositories. On the other hand, larger energy changes are studied by inelastic neutron scattering (INS). These experiments reveal vibrational modes which are not subject to selection rules, unlike optical spectroscopy. The observed intensities, across all neutron scattering techniques, can be readily modelled by computer simulation. This enables researchers to put forward plausible explanations that fit experimental data, such as the postulation of crystal defects and distinct molecular motions. Finally, during sample preparation, H/D isotopic substitution can enable contrast matching, a useful tool that can ‘highlight’ or ‘hide’ different hydrogen-containing features in clay minerals.

Figure 1. Schematic of information that can be obtained from neutron scattering studies on clay minerals. Background image: ISIS Neutron and Muon Source, UK

Aasim Shaffi

Department of Physics & Astronomy

University College London (UCL)

Read the full newsletter here – CMG-newsletter-Jubilee-edition-issue-1.pdf (minersoc.org)

Ryan Bower

I really enjoyed my time as a member of the CDT-ACM. It provided the opportunity to gain experience with a wide range of materials characterisation techniques and to engage with the wider research community by participating in conferences and Outreach events.