Large elastic recovery of zinc dicyanoaurate

We report the mechanical properties of the `giant' negative compressibility material zinc(II) dicyanoaurate, as determined using a combination of single-crystal nanoindentation measurements and \emph{ab initio} density functional theory calculations. While the elastic response of zinc dicyanoaurate is found to be intermediate to the behaviour of dense and open framework structures, we discover the material to exhibit a particularly strong elastic recovery, which is advantageous for a range of practical applications. We attribute this response to the existence of supramolecular helices that function as atomic-scale springs, storing mechanical energy during compressive stress and hence inhibiting plastic deformation. Our results are consistent with the relationship noted in [Cheng \& Cheng, \textit{Appl. Phys. Lett.}, 1998, {\textbf{73}}, 614] between the magnitude of elastic recovery, on the one hand, and the ratio of material hardness to Young's modulus, on the other hand. Drawing on comparisons with other metal--organic frameworks containing helical structure motifs, we suggest helices as an attractive supramolecular motif for imparting resistance to plastic deformation in the design of functional materials.