When it comes to determining the fundamental mechanical response of framework materials, it seems that topology — rather than chemistry — plays the deciding role.
Understanding the ways in which material structures change with temperature or pressure is a crucial step in exploiting functional properties such as ferroelectricity, piezoelectricity, or crystalline-amorphisation switching. In particular, the existence and nature of phase transitions tell us what types of atomic motion are most relevant to the physics of a given material — this is crucial information because in designing functional materials we need to know not just where the atoms are, but how they are moving.
While most studies of phase transitions in framework materials have focussed on oxide-based ceramics, there are an increasingly large number of cases where molecular frameworks — such as the ubiquitous metal–organic frameworks (MOFs) — have been found to exhibit an equally rich variety of critical behaviour. If we are to translate these discoveries into a set of usable design principles, however, we need to understand what are the “rules” which govern the underlying dynamical response of molecular frameworks. At face value, this seems like an incredible challenge given the enormous chemical versatility of MOFs.
In a recent study of critical behaviour in molecular frameworks, we have concentrated on the structural changes involved in two frameworks of very different chemistries: namely, zinc cyanide (Zn(CN)2) and cadmium imidazolate (Cd(im)2). Their only similarity lies in the connectivity of the frameworks, which both have diamondoid topologies. By looking for similarities and differences in their mechanical properties, we are effectively answering the question of whether topological similarity is sufficient to define a common mechanical response, or whether framework chemistry must also be taken into account.
To probe the mechanics of the two molecular frameworks, we performed variable-pressure and variable-temperature crystallographic experiments. What we find is that the same structural changes occur for zinc cyanide with increasing pressure as occur for cadmium imidazolate under decreasing temperature. This means that the dominant mechanical response of both systems is dominated by the framework topology, and are essentially chemistry independent. So we can now look to predicting the functional behaviour of molecular frameworks from their topology alone. The role of chemistry becomes one of tuning the magnitude and rate of response.