Inorganic Network Structures Exhibiting Unusual Negative Behaviours

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Most everyday materials will become thinner when stretched: for example, consider stretching an elastic band - you would picture the strip of rubber becoming narrower. Some, less common, materials behave rather differently, and when stretched actually become fatter: these materials are described as being auxetic. This is not just a scientific curiosity as auxetic materials have many technological applications in areas ranging from toughened engineering materials to nanotechnology (for example, an auxetic solid might act as a tuneable sieve on a molecular scale, because the network could expand to produce an open structure). Auxetic materials may be described as having 'negative behaviour': behaviour which is counterintuitive. A second type of negative behaviour is 'negative thermal expansion'. Here a material shrinks when it is heated, rather than expanding, as might be expected. As with auxetic behaviour, negative thermal expansion has many technological applications, for example, in making composite materials that show no expansion when heated. It is believed that the same fundamental processes can be responsible for these two negative behaviours. This, however, has not yet been proved, and in order to do so, experimental measurements must be made that probe both the structure of matter on an atomic scale, and the properties of large specimens, to relate the bulk properties of materials to atomic structure.Our proposal is to make these experimental measurements for the first time. We will focus upon one family of materials: the open-network silicates. These materials have been predicted by computer simulations to be auxetic, but this prediction has never been verified, although some also are known already to have negative thermal expansion properties. The silicates are commonly found as minerals in the Earth's crust, but in the laboratory we are able to make large crystals of the materials in a pure state; this is something that in the past has been difficult to do, but we have recently developed reliable methods to do so. The large crystals will be subjected to mechanical tests - literally stretching the specimens along different directions will allow us to measure their elastic properties and determine whether the proposed negative behaviour exists. Since we can study a large number of specimens with differing chemical compositions, we will be able to relate the chemistry of the material to the elastic properties, and then determine relationships between the of chemical composition of sample (that is atomic-scale properties) and its bulk elastic properties. We aim to use these results to verify computer predictions of auxetic behaviour and then understand why auxetic behaviour exists. The materials will also be studied for their thermal expansion properties: the question to address here is whether solids that possess auxetic behaviour also show negative thermal expansion. This has never been studied before. The proposed work is a collaboration between chemists and engineers and this interaction will be an ideal combination to achieve our aims: the chemists are experts in making new materials and understanding their structures, while the engineers study the elastic and mechanical properties of materials.