Mechanical and configurational degeneracy in transition metal cyanide materials
Keywords: Flexibility
Disordered Materials
Cyanide
Mechanical Properties

Molecular framework materials - materials constructed like atomic scaffolding where anionic polyatomic linkers join cationic metal nodes - have great currency in contemporary functional material design. In this thesis the structures and properties of transition metal cyanides are investigated, focusing on the impact of mechanical and configurational degeneracy. These related phenomena give rise to materials with unusual and extreme negative mechanical responses on the one hand, and exotic structural states on the other.
The mechanical response of primary interest here is negative linear compressibility (NLC), a rare but desirable property where a material actually expands in one direction against increasing hydrostatic pressure. The performance of NLC materials|for use in areas such as sensitive pressure sensing, actuators, and development of artificial muscle|critically depends on the intrinsic NLC response. Until recently very few NLC materials were known and, of those, the expansion was small in comparison to the normal contraction of common engineering materials.
Here I show that several dicyanometallate frameworks, those based on the almostlinear [M(CN)2]- (M = Ag, Au) linker, exhibit a prolonged NLC response up to ten times larger than established NLC candidates. Using two structure design modifications - soft-mode frustration to prevent structure collapse and supramolecular motifs that enhance compressibility - both the range over which NLC is observed and the magnitude of response have been increased. In the former, the extended perovskite-like system KM[M'(CN)2]3 is used as a model system, whilst 'spring'-like supramolecular interactions in zinc(II) dicyanoaurate(I), Zn[Au(CN)2]2, gives rise to an exceptional response - the first report of 'giant' NLC.
Variable-pressure studies on the canonical isotropic negative thermal expansion (NTE) material zinc (II) cyanide reveals both its intrinsic phase transition behaviour, giving insight into the mechanism of NTE, and NLC as a result of the improper ferroelastic phase transition at 1.52GPa. This investigation resolves several discrepancies and contradictions in the field and suggests the mechanical responses of isostructural frameworks may be understood in terms of underlying dynamic instabilities, in much the same way as for many conventional ceramic materials.
In the second part of this thesis, configurational degeneracy is explored focusing on structural disorder. Beginning with a summary of well-characterised examples of disorder in molecular frameworks, I go on explore the collective states arising from degeneracy in the 'simple' chain cyanides AgCN, AuCN and (Ag1/2Au1/2)CN. By considering chains as geometric objects with simple pairwise interactions, we find the emergence of complex structural states that are directly related to the XY magnetic structures on a triangular lattice. Structurally degenerate states appear inherent in a range of framework materials; many possibilities exist in this field for targeting, enhancing and coupling counterintuitive mechanics and complex states with other functionalities.