Single crystals as pistons – expansion in only one direction upon heating

March 29, 2018
Figure: A simplified “ball and stick” illustration showing two layers of the crystal structure of a coordination polymer designed by the Leznoff lab; the arrows indicate the direction in which the lattice expands when heated. The background design shows the X-ray diffraction pattern of the polymer.

The motivation – The Leznoff Group at Simon Fraser University studies how metal-based polymers (coordination polymers) respond to external stimuli such as changes in temperature. To understand the specific properties of a material, a researcher first needs to know its chemical make-up and how its components are arranged with respect to one another. Then, with a thorough understanding of a class of materials, new versions can be developed by modifying components in ways that give the coordination polymer a specific property (e.g., porosity, magnetism).  Key problems to consider are “Why does a particular material change in a certain way when an external stimulus is applied?” and “How can a material be engineered to have specific properties to suit desired applications?” In particular, the Leznoff group is interested in the effect of temperature changes on materials, which is a critical factor in the selection of components to be used in specific applications (e.g., from technological devices to buildings). Usually, heating a three-dimensional material causes it to expand in all directions – a response that is called “positive thermal expansion.” And depending on the structure and composition, materials will have different thermal expansion properties.

This study aimed to combine components with known structural preferences and thermal expansion properties for the rational design of new materials. Leznoff Group members Ania Sergeenko and Dr. Jeff Ovens designed coordination polymers that would combine metals and link units to form materials that, upon heating, would expand in only one direction, acting as a piston.

The discovery – Taking advantage of the fact that platinum favours the formation of materials that propagate in two-dimensional sheets and that platinum/cyanide-based systems have been shown to exhibit zero thermal expansion, they designed a material that would form 2-D sheets with zero thermal expansion. And, given that platinum can be modified such that additional units can be attached to stick in and out of the sheet, occupying the third dimension, they selected a unit that would hold sheets together weakly and show extremely large positive thermal expansion in the inter-sheet direction. The coordination polymers were synthesized and their supramolecular arrangement and structural changes with temperature were probed at the molecular level by X-ray diffraction analysis. The results demonstrated that the materials responded to temperature changes as anticipated, showing zero or near zero thermal expansion in two dimensions and colossal thermal expansion in the third dimension.

Its significance –Overall, the inherent properties of platinum were used to rationally design materials that act as pistons by expanding in only one direction. These findings show how coordination polymers can be used to design materials that move in pre-defined ways. Future work will focus on how further manipulation of the platinum centre affects its structure and thermal response.

Read the paper – “Designing anisotropic cyanometallate coordination polymers with unidirectional thermal expansion (TE): 2D zero and 1D colossal positive TE” by Sergeenko, AS; Ovens, JS; Leznoff, DBChemical Communications 54(13):1599-1602 (2018). DOI: 10.1039/c7cc09033a

Website article compiled by Jacqueline Watson with Theresa Kitos