Spectra (TOSCA) at 100 K (red) and 20 K (blue) for Cu3(1,3,5-benzenetricarboxylate)2. Low energy features arise from the dicopper benzoate unit (inset along Cu-Cu) which changes from square-prismatic (left) to distorted (right).
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Negative thermal expansion (NTE), or contraction upon heating, is of fundamental scientific interest and may find applications in precision engineering.
Understanding the mechanisms for NTE may allow tuning of the behaviour by modification of the atomic-level structures through which it is produced. A principal cause of NTE is transverse atomic and molecular vibrations – vibrations that are at right angles to the atomic or molecular bonds. Such vibrations are often low in energy, making them difficult to model and therefore identify. However, the Tosca spectrometer can now be run with its chopper stopped during the one pulse in five that goes to the ISIS Second Target Station, enabling collection of the lower energy region of the vibrational spectrum.
NTE is being noted increasingly in metal-organic framework materials, which have been studied principally for applications such as gas storage. Using Tosca we have found a unique and relatively complex mechanism for NTE in one such framework, Cu3(1,3,5-benzenetricarboxylate)2. We find NTE arises from relief of geometric frustration through motions of its dicopper benzoate unit, which distorts from square-prismatic towards the antiprismatic configuration on heating.
VK Peterson, GJ Kearley (ANSTO, Australia), Y Wu (Sydney University, Australia), AJ Ramirez-Cuesta (ISIS), CJ Kepert (Sydney University, Australia)
Contact: Dr Vanessa Peterson, Vanessa.Peterson@ansto.gov.au
Further reading: VK Peterson et al., submitted.
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