Dr Arnaud Marmier, Exeter

Negative Poisson's ratio and other unusual elastic properties in pure silica zeolites: a DFT and force field study
When Mar 09, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
Contact Name
Contact Phone 01865-273925
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Poisson’s ratio can be a complex, highly anisotropic property that can have interesting and useful negative values. This phenomenon, referred to as auxeticity, is usually limited to a small range of directions, but can occasionally occur in a larger range, or even for all directions. Beside the minimum value there are presently no indicators of the “amount” of auxeticity of a material, and we propose a carefully derived typology to overcome this limitation. To establish a ”base-line” for auxeticity, we characterise the auxeticity of numerous single crystals using previously published elastic tensors. We demonstrate that occasional auxeticity occurs in around 37% of the cases, while “average” auxeticity is much less prevalent, and limited to the silica α-cristobalite. With the help of this scheme, we consider pure silica zeolites, previously thought of as good candidates due to their low density and chemical similarity to cristobalite. Due to the lack of experimental elastic data for pure silica zeolite (limited to MFI), we calculate the elastic tensors with Density Functional Theory and several force-fields methods. We find that the different models are in excellent agreement and that on average pure silica zeolite are not more auxetic than the reference crystals. On the other hand, we also identify with great confidence that the framework JST is not only averagely auxetic, but in fact completely auxetic, the first crystal with this property. Finally, we propose a mechanism, based on stiff bonds and flexible angles, which mimics the behaviour of JST under stress and explain its generalised auxeticity.