A new technique will freeze materials to temperatures near absolute zero in milliseconds, trapping their atoms in unusual arrangements that would otherwise collapse into a more stable form. Many advanced materials—from the glass in fibre-optic cables to the alloys in jet engines—are metastable: they exist in a state that is not their lowest-energy configuration. Researchers have long used rapid cooling to create such materials at high temperatures, but the same approach has been impossible at cryogenic temperatures, where many technologically relevant transitions occur. This project solves that problem by developing sample holders that can cool materials to liquid helium temperatures faster than the atoms can rearrange, and using Diamond Light Source’s X-ray beam to watch the atomic structure in real time. If successful, the work could unlock a new class of negative thermal expansion materials—substances that shrink when heated and expand when cooled, the opposite of normal behaviour. These materials could solve a persistent engineering headache: thermal cycling, which causes components in satellites, fuel cells, and precision instruments to expand and contract, leading to fatigue and failure. Beyond that, the rapid-cooling technique itself could be applied to study other low-temperature phenomena, from superconductivity to magnetism, opening a general route to discovering metastable materials that have never been made before.
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Technologically important materials are increasingly not the most stable arrangement of atoms. Instead, useful materials such as glass or complex alloys are often metastable, meaning they are somehow prevented from transitioning to a more stable state. One method to achieve this is through quickly cooling a sample, freezing the metastable structure before it can undergo a phase transition. This approach is common at high temperatures, but has not been exploited for transitions occurring below room temperature due to technical challenges with rapid, cryogenic cooling. Given the importance of low temperature materials in areas such as space or quantum computing, a low-temperature route to discovering advanced materials would have clear benefits. This project aims to solve the challenges of rapid cooling to liquid helium temperatures, and use the resulting technology to accelerate discovery of new metastable materials. The optimum approach to rapidly discover new metastable materials is to directly determine the atomic structure during rapid cooling. Synchrotron X-ray diffraction provides the perfect tool for this, allowing crystalline structure to be measured with millisecond time resolution. Diamond Light Source is one of very few synchrotrons with a cryostat having the potential for rapid cooling while simultaneously measuring diffraction, but currently this technique is unavailable. This collaboration seeks to achieve the technical developments required for such measurements, and use this approach to discover new advanced materials. The first objective of the project is to develop sample holders for diffraction with the high thermal conductivity required for rapid cooling, and precisely characterise the cooling behaviour using in-situ diffraction. The second objective is then to use rapid cooling to quench structural phase transitions below room temperature. The class of materials chosen are those with the rhenium trioxide structure, many of which show the unusual behaviour of expanding as they are cooled. The reason for choosing these materials is that there is a direct competition between this negative thermal expansion behaviour and a structural phase transition; if the phase transition can be avoided by rapid freezing, there is a high chance that a negative thermal expansion material will result. As well as being of fundamental scientific interest, these materials offer a materials-based solution to thermal cycling in applications ranging from satellites to fuel cells. The impacts of this project will be threefold; firstly, the technological developments which enable rapid cooling could enable new discoveries for other low temperature transitions, from superconductivity and charge ordering to ferroelectricity and magnetism. Secondly, a large number of new negative thermal expansion materials will be discovered, although such materials have also seen interest as battery electrodes. Finally, discovering that low temperature structural transitions can be quenched could readily be applied to other materials, opening up a plethora of new low-temperature materials and their physical properties.
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