The paper reports on the study into the very low-temperature behaviour of the most commonplace and familiar variety of ice, known as ice lh. Despite its familiarity, there remain many aspects of the crystal structure of ice that are unknown. One such aspect is the way in which water molecules in the crystal, which are oriented randomly near the melting point of ice, arrange themselves into highly organised orientations at low temperature.
This process, called 'ordering', occurs at −200°C but evidence about the ordering process on cooling down to −200°C has been difficult to obtain since the signature is subtle and the process is slow. The researcher, beamline scientist Dominic Fortes, found that the thermal expansion of the ice crystal along different directions could be used as a proxy 'indicator' of how much partial ordering took place as ice was heated and cooled at different rates.
Through a series of measurements taken at one of the highest resolution neutron powder diffraction instruments in the world, the HRPD beamline here at ISIS, Dominic was able to measure the very small differences of thermal expansion in ice samples that had been prepared in different ways – for example, slow 'natural' freezing versus flash-freezing – and then subjected to different experimental heating and cooling procedures.
In particular, the shape of the crystal's fundamental 'building block', the so-called unit-cell, changed in a highly reproducible fashion that indicated the degree or order in the orientations of the water molecules. How the shape deviated from this behaviour could be controlled by very rapid cooling, allowing specific degree of partial order to be frozen-in. This is important since it allows for other types of measurement to be made that may corroborate Dominic's interpretation.
This study has shed new light on the value of using the technique of neutron powder diffraction to study the dynamic process behind the structures formed in these types of materials, an in particular the value of very high-resolution diffraction to obtain the highest precision on structural properties.
Further information
The full article can be read on the Physical Chemistry Chemical Physics webpage, and the 2019 PCCP HOT articles can be found here.
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