Spin crossover-induced colossal positive and negative thermal expansion in a nanoporous coordination framework material

Abstract

External control over the mechanical function of materials is paramount in the development of nanoscale machines. Yet, exploiting changes in atomic behaviour to produce controlled scalable motion is a formidable challenge. Here, we present an ultra-flexible coordination framework material in which a cooperative electronic transition induces an extreme abrupt change in the crystal lattice conformation. This arises due to a change in the preferred coordination character of Fe(II) sites at different spin states, generating scissor-type flexing of the crystal lattice. Diluting the framework with transition-inactive Ni(II) sites disrupts long-range communication of spin state through the lattice, producing a more gradual transition and continuous lattice movement, thus generating colossal positive and negative linear thermal expansion behaviour, with coefficients of thermal expansion an order of magnitude greater than previously reported. This study has wider implications in the development of advanced responsive structures, demonstrating electronic control over mechanical motion.

Molecular Spin Crossover Materials: Review of the Lattice Dynamical Properties

“…As a final remark, we would like to mention that, in certain materials, the SCO phenomenon is concomitant with other interesting lattice dynamical phenomena, such as negative thermal expansion and negative linear compression . Since all these phenomena are intimately linked to low‐frequency lattice phonons, we feel that investigating the possible interplay between them may be mutually beneficial for these different fields.…”

A survey of thermal expansion coefficients for organic molecular crystals in the Cambridge Structural Database

“…The survey excludes metal complexes, for which temperaturedependent electronic transitions often influence thermal expansion (e.g. Buron Le-Cointe et al, 2012;Mullaney et al, 2017), and framework structures (MOFs), for which thermal expansion is highly relevant (e.g. Goodwin et al, 2008;Phillips et al, 2008;Cliffe et al, 2015) but often controlled by covalent bonds rather than non-bonded intermolecular interactions.…”

Tuning of High Spin Ground State and Slow Magnetic Relaxation within Trimetallic Cyanide‐Bridged {NiIIxCoII9−x[WV(CN)8]6} and {MnIIxCoII9−x[WV(CN)8]6} Clusters

“…In light of the above solutions, the formation of multi‐metallic coordination compounds is expected to multiply the opportunities to achieve not only simple, but also combined or even interacting functions. Along this line, very elegant examples in the field of coordination chemistry were shown using the “simple” mixing strategy, solid‐solution strategy and modular strategy,, yielding novel species of tunable magnetic properties, for example, magnetic sponge‐like behaviour, slow relaxation of the magnetization and spin‐crossover, as well as negative thermal expansion and luminescence. Being strongly interested in various structural and functional aspects of polymetallic complexes and networks, we have recently described a unique series of solid‐solutions constructed of trimetallic cyanide‐bridged {Fe II 9− x Co II x [W V (CN) 8 ] 6 (MeOH) 24 } ( FeCoW ) clusters ( x= 1, 2,…, 7, 8; all values of x approximating real compositions) .…”