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.
Highlights
External control over the mechanical function of materials is paramount in the development of nanoscale machines
It is well understood that most materials exhibit positive thermal expansion (PTE) as higher temperatures increase the amplitude of atomic bond vibrations[6], for which the relative rate of thermal expansion[7], α, usually lies within the range 0 × 10−6 K−1 < α < 20 × 10−6 K−1
We exploit the electronic phenomenon of spin crossover, which is very strongly coupled to the crystal lattice, to achieve unprecedented thermal mechanical function, as observed through extreme PTE and negative thermal expansion (NTE)
Summary
External control over the mechanical function of materials is paramount in the development of nanoscale machines. 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. The strong electron–lattice coupling is due to changes in the geometry and strength of coordination bonding at the metal site Exploiting this structure–property relationship, recent reports have shown that spin crossover materials can be used to create light-induced molecular actuators[20, 21]. Strategically diluting the framework with Ni(II) disrupts the cooperativity of the spin crossover, resulting in continuous colossal thermal expansion over the transition temperature range
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