Colossal Positive and Negative Thermal Expansion in the Framework Material Ag 3 [Co(CN) 6 ]

Abstract

We show that silver(I) hexacyanocobaltate(III), Ag 3 [Co(CN) 6 ], exhibits positive and negative thermal expansion an order of magnitude greater than that seen in other crystalline materials. This framework material expands along one set of directions at a rate comparable to the most weakly bound solids known. By flexing like lattice fencing, the framework couples this to a contraction along a perpendicular direction. This gives negative thermal expansion that is 14 times larger than in ZrW 2 O 8 . Density functional theory calculations quantify both the low energy associated with this flexibility and the role of argentophilic (Ag + ...Ag + ) interactions. This study illustrates how the mechanical properties of a van der Waals solid might be engineered into a rigid, useable framework.