A three-step change in uniaxial negative thermal expansion by switching supramolecular motion modes in a ferromagnetically coupled nickel dithiolate lattice†

The wheel–axle-type supramolecule ((⁺H₃N–C₂H₄)₂O)([18]crown-6)₂ was introduced into a crystal as a counter cation of [Ni(dmit)₂]⁻. Within the crystal, [Ni(dmit)₂]⁻ was arranged in a honeycomb-like structure and one-dimensional chains formed by ((⁺H₃N–C₂H₄)₂O)([18]crown-6)₂ penetrated each honeycomb lattice. In the supramolecular structure, (⁺H₃N–C₂H₄)₂O behaved as a rigid axle. Two [18]crown-6 molecules, CE1 and CE2, behaving like wheels, were crystallographically independent of each other, with CE2 showing disorder. The crystal exhibited uniaxial negative thermal expansion (NTE). With increasing temperature, the disordered state of CE2 changed, resulting in a change in the coefficient of linear thermal expansion (CLTE) of the crystal in all directions. In the X₁ direction, where NTE was observed, the CLTE was relatively small at −18 × 10⁻⁶ K⁻¹ in phase I (113–133 K) in which CE2 was static. In phase II (143–193 K), when CE2 starts to rotate, the CLTE of the NTE increases significantly to −74 × 10⁻⁶ K⁻¹. Furthermore, in phase III (213–293 K), where [18]crown-6 out-of-plane motion is also involved, the NTE is extremely large with a value of −105 × 10⁻⁶ K⁻¹. Ferromagnetic interactions between [Ni(dmit)₂]⁻ molecules were observed, with the Weiss temperature of +1.41 K. Multi-step changes in the mode of motion in supramolecular structures may offer new possibilities for multi-step control of the thermal expansion of crystals.