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arXiv · 1702.07133

Negative thermal expansion and metallophilicity in Cu$_3$[Co(CN)$_6$]

Adam F. Sapnik·Xiaofei Liu·Hanna L. B. Boström·Chloe S. Coates·Alistair R. Overy·Emily M. Reynolds·Alexandre Tkatchenko·Andrew L. Goodwin
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Abstract

We report the synthesis and structural characterisation of the molecular framework copper(I) hexacyanocobaltate(III), Cu$_3$[Co(CN)$_6$], which we find to be isostructural to H$_3$[Co(CN)$_6$] and the colossal negative thermal expansion material Ag$_3$[Co(CN)$_6$]. Using synchrotron X-ray powder diffraction measurements, we find strong positive and negative thermal expansion behaviour respectively perpendicular and parallel to the trigonal crystal axis: $α_a$ = 25.4(5)\,MK$^{-1}$ and $α_c$ = $-$43.5(8)\,MK$^{-1}$. These opposing effects collectively result in a volume expansivity $α_V$ = 7.4(11)\,MK$^{-1}$ that is remarkably small for an anisotropic molecular framework. This thermal response is discussed in the context of the behaviour of the analogous H- and Ag-containing systems. We make use of density-functional theory with many-body dispersion interactions (DFT+MBD) to demonstrate that Cu$\ldots$Cu metallophilic (`cuprophilic') interactions are significantly weaker in Cu$_3$[Co(CN)$_6$] than Ag$\ldots$Ag interactions in Ag$_3$[Co(CN)$_6$], but that this lowering of energy scale counterintuitively translates to a more moderate---rather than enhanced---degree of structural flexibility. The same conclusion is drawn from consideration of a simple lattice dynamical model, which we also present here. Our results demonstrate that strong interactions can actually be exploited in the design of ultra-responsive materials if those interactions are set up to act in tension.

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Adam F. Sapnik, Xiaofei Liu, Hanna L. B. Boström, Chloe S. Coates, Alistair R. Overy, Emily M. Reynolds, Alexandre Tkatchenko, Andrew L. Goodwin. 2017-02-23. Negative thermal expansion and metallophilicity in Cu$_3$[Co(CN)$_6$]. https://doi.org/10.1016/j.jssc.2017.10.009

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