Diverse defects in metal thiocyanate frameworks

“Your imperfections are what make you perfect” is not just a twee word-art slogan on the wall of a chain coffee shop but also a guide for materials chemistry. Defects, the deviations from perfect crystalline order, are key to producing the useful properties of many solids, from solar cells to gemstones. The challenge facing chemists, however, is that defects are hard to see: there are not many of them and they are usually randomly distributed throughout the material. This means that standard crystallographic techniques, which rely on long-range order, are much less useful. As a result, it is very challenging to know what defects are present in a material, even if we can see their consequences.

Hybrid frameworks, inorganic solids containing molecular components, are a particularly exciting class of materials at the moment because they can have properties unknown in purely inorganic solid materials, due to their increased flexibility. Chemists have recently started taking advantage of defects to enhance the properties of these materials, but we still have only very limited information about defects in hybrid frameworks look like. In our group we have been investigating frameworks made using thiocyanate (NCS^–) as a linker because it produces intense colours and can transmit magnetic interactions between the metal ions it connects. We found that we could generate frameworks with large numbers of defects which, because of the low symmetry of these materials, were also long-range ordered. This allowed us to use standard single-crystal diffraction experiments to determine the structures of the defects on an atomic level. We found, to our surprise, a wide range both of defects and defect-ordering patterns. The diversity we observed suggests that there maybe be similar diversity in other defective hybrid frameworks which has thus far escaped detection.

These gifs show the distortions (octahedral tilt patterns) present in three of the thiocyanate frameworks (1,2 and 4) from a hypothetical high-symmetry structure