• Sapnik, Af
• Goodwin, A. L.
• Liu, X.
• Boström, H. L. B.
• Coates, C. S.
• Overy, A. R.
• Reynolds, E. M.
• Tkatchenko, A.

Abstract

<p>We report the synthesis and structural characterisation of the molecular framework copper(I) hexacyanocobaltate(III), Cu₃[Co(CN)₆], which we find to be isostructural to H₃[Co(CN)₆] and the colossal negative thermal expansion material Ag₃[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⁺…Cu⁺ metallophilic (‘cuprophilic’) interactions are significantly weaker in Cu₃[Co(CN)₆] than Ag⁺…Ag⁺ interactions in Ag₃[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 GULP 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.</p>

Topics

• density
• impedance spectroscopy
• dispersion
• theory
• anisotropic
• copper
• thermal expansion
• density functional theory
• Ag-containing