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 905 K Pnma-Imma phase transition in SrSn O3 is studied here using a combination of variable-temperature neutron total scattering together with the reverse Monte Carlo (RMC) refinement method. The real-space RMC configurations obtained are analyzed in terms of bond distance and bond-angle distributions, and a geometric algebra approach is used to quantify the associated octahedral-tilting distributions. What emerges from this analysis is that the transition is displacive in nature, in contrast to the results of a recent average-structure investigation in which an order-disorder model was proposed. Three-dimensional diffuse scattering patterns calculated from the same RMC configurations reveal the existence of an additional disorder mechanism which persists across the Pnma-Imma transition. The "reflection conditions" of this diffuse scattering, together with displacement correlation calculations, point to the existence of ferroelectric nanoscale domains within the configurations, which are found to extend across planar regions of approximately 10-15 Å in diameter. © 2007 The American Physical Society.