THE JOY OF RULEBREAKING

Just like a water bottle shrinks at the end of a flight as the air pressure increases, most objects get smaller in every direction when pressure is applied equally (hydrostatically) around it. Negative linear compressibility (NLC) is the bizarre phenomenon that involves a structure actually expanding in one direction under the same conditions — in doing so it has to fight against the external pressure.

For some time our design approach to engineering NLC has been to focus on materials that show strong and anisotropic negative thermal expansion (NTE). The working assumption here is that two phenomena share the same underlying geometric origin: our (clichéd?) analogy here is that frameworks can act like a collapsible wine-rack. On heating, the rapid expansion of weak bonds is translated by this geometry into an equally large collapse in a perpendicular direction; on compression, the situation is simply reversed as the weak bonds are the most easily deformed.

In order then to enhance negative compressibility — so the thinking goes — we simply have to design a material that is as positively compressible as possible in one set of directions, which can then be transferred through the framework to a negative response. Here we turn to engineering for inspiration: springs are the design motif used in situations when compression must be maximised. In the paper below, we have shown that zinc(II) dicyanoaurate(I) contains supramolecular ‘springs’ assembled from weak gold…gold interactions. These springs do indeed render the material highly compressible in two directions, that, through the framework topology, gives rise to the largest negative response ever reported.

Working as we do with materials that behave in an opposite sense to what intuition demands — shrinking when they are heated, or expanding when they are squeezed — means that we are becoming used to expecting the unexpected in much of our work. Luckily it appears that there are still surprises to be found — even in this world of counterintuitive response. And it is through studying these surprises that we hope to uncover new opportunities in material design.

Giant negative linear compressibility in zinc dicyanoaurate. Nature Materials 12, 212-216 (2013)

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