NEGATIVE THERMAL EXPANSION IN CYANide-Bridged coordination FRAMEWORK Materials

Karena W. Chapman, Andrew L. Goodwin, and Cameron J. Kepert

School of Chemistry, University of Sydney, NSW 2006, Australia (chapmank@chem.usyd.edu.au)

The current interest in coordination framework materials derives from the wide range of possible functionalities that can be achieved and the exciting potential to tailor these properties towards specific applications through systematic variation of metal ion, ligand and included guest species. Reported functionalities have included those of a photochemical, mechanical, optical, electronic and magnetic nature, allowing applications in selective sorption, catalysis, sensing, switching and data storage.

We have synthesised and characterised a versatile family of cyanidebridged coordination framework materials that display negative thermal expansion (NTE) behaviour: contracting upon heating. These have diverse potential applications in high precision and low thermal shock materials (in zero thermal expansion composites) where the positive thermal expansion exhibited by the vast majority of materials may be a hindrance.

The thermal expansion in these frameworks have been monitored with variable temperature single crystal and synchrotron powder x-ray diffraction, recording coefficients of thermal expansion (a_ℓ = dℓ/dT) up to 21 x 10⁶ K¹, more than twice that of the previous record held by zirconium tungstate.

This NTE behaviour arises from a contraction of M…M´ distances due to the increased amplitude of M-CN-M´ transverse vibrations at high temperature. This is crystallographically evident in the atomic displacement parameters of the cyanide ligand perpendicular to the M…M´ axis as shown in the single crystal structural models and powder neutron diffraction.