David Keen

Abstract:

We show that silver(I) hexacyanocobaltate(III), Ag3[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 ZrW2O8. 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.

Abstract:

The ionic diffusion mechanism of mobile ions within an underlying body-centred cubic (bcc) sublattice of immobile counterions is discussed. In particular, the case of equal numbers of two ionic species forming long-range ordered and disordered bcc arrays is considered, since these form the basis of the cubic perovskite and α-AgI-type crystal structures, respectively. Their structural behaviour, and its influence on the dynamic ionic disorder which characterizes superionic conduction, is illustrated for the case of Ag(+) diffusion within the β and α(*) phases of Ag(3)SI. The calculated behaviour obtained by molecular dynamics (MD) computer simulations is validated with reference to published neutron diffraction and ionic conductivity measurements of Ag(3)SI, and used to examine the preferred diffusion pathways. The relevance of these findings for the anion conduction mechanisms within perovskite structured compounds is briefly discussed.