Negative thermal expansion (NTE) involves the unconventional phenomenon of material contraction upon heating and understanding material thermal expansion behaviour remains an important engineering consideration. Our detailed investigation on the fundamental mechanisms of thermal expansion endeavours to facilitate the process of engineering metal-organic framework (MOF) materials to address sustainability challenges.
Our research aims to contribute knowledge to the following two pillars:
1. Structure-property relationships – how does the framework structure alter the mechanisms leading to negative thermal expansion?
2. Host guest interactions – how do guest interactions affect thermal expansion?
To instigate the structure-property relationship, we explored the thermal expansion properties of two different MOF families, both constructed from basic zinc acetate clusters and carboxylate linkers with either a pcu or ith-d topology. Powder X-ray diffraction studies reveal an enhanced NTE in the aliphatic MOFs compared with the aromatic analogues. Single crystal X-ray diffraction revealed the local mechanisms governing NTE in both materials to be rigid unit modes and transverse linker vibrations. This was corroborated using Far-IR, performed at the Australian Synchrotron and also computational calculations. Both these techniques allow the phonon modes of vibration to be studied, giving the underlying framework motion. In extension, we sought to exploit host-guest interactions to attain tuneable NTE, by charging 3DL-MOF-1 with CO2 guest molecules. Through using Neutron Powder Diffraction (NPD) we demonstrate successful NTE quenching by increasing the CO2 loading into the framework. The holistic comprehension of structure-property relationships relating to aliphatic linkers and their host-guest variations highlight the versatility of this class of materials to become the next generation of tunable functional materials.