Cobalt adipate, Co(C6H8O4): antiferromagnetic structure, unusual thermal expansion and magnetoelastic coupling

Co adipate, Co(C6H8O4), has been found to order near 10 K into a magnetic structure featuring sheets of tetrahedral Co cations coupled antiferromagnetically in two dimensions through carboxylate groups. The emergence of this order is accompanied by magnetoelastic coupling, which drives anisotropic negative thermal expansion along the a-axis below 50 K, the first time such behaviour has been observed in a metal-organic framework. The monoclinic angle, beta, has also been found to decrease on cooling, passing through a metrically orthorhombic phase without a phase transition; this unusual behaviour has been rationalised in terms of the thermal expansion along the principal axes.