Uniaxial negative thermal expansion and metallophilicity in Cu3[Co(CN)6]

Journal article

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 colossalnegative 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 thetrigonal crystal axis:α= 25.4(5) MKa−1andα= − 43.5(8) MKc−1. These opposing effects collectively result in avolume expansivityα= 7.4(11) MKV−1that is remarkably small for an anisotropic molecular framework. Thisthermal 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) todemonstrate 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 translatesto a more moderate—rather than enhanced—degree of structural flexibility. The same conclusion is drawn fromconsideration of a simple GULP model, which we also present here. Our results demonstrate that stronginteractions can actually be exploited in the design of ultra-responsive materials if those interactions are set upto act in tension.

Authors: Sapnik, A; Liu, X; Boström, H; Coates, C; Overy, A

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Elsevier
• Journal: Journal of Solid State Chemistry
• Volume: 258
• Pages: 298-306
• Publication date: 2017-10-07
• Acceptance date: 2017-10-05
• DOI: 10.1016/j.jssc.2017.10.009
• EISSN: 1095-726X
• ISSN: 0022-4596
• Keywords: negative thermal expansion; framework materials; metallophilic interactions