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Metal-organic frameworks (MOFs) have recently attracted great attention for their multiferroic, magnetocaloric and low dimensional magnetic order. These properties depend on the precise magnetic interactions in frameworks and a deeper understanding of the coupling between the lattice of these materials and their magnetic order is required to underpin the future developments of these promising compounds. Neutron diffraction is the ideal technique for such studies but its application to studies of the magnetic structure of MOFs has been limited to date due to challenges in solving the structure of these dilute yet complex magnetic compounds. We have recently examined the magnetic properties and structure of a new dicarboxylate framework, cobalt adipate, Co(C6H8O4), which adopts P21/c monoclinic symmetry.[1] This compound has been found to order antiferromagnetically at low temperature and fits to neutron diffraction data have shown it adopts Pb21/c magnetic symmetry. Its magnetic structure features sheets of 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 at low temperature. This is the first evidence for such behaviour in a MOF and we find that both this behaviour and the spin orientation in this material are controlled by the presence of weak ferromagnetic dipole-dipole coupling between the layers of tetrahedral cobalt. Variable temperature high resolution synchrotron X-ray and neutron powder diffraction have also revealed that the monoclinic angle of Co adipate decreases on cooling, passing through a metrically orthorhombic phase without any indication of a phase transition. This unusual behaviour has been rationalised by examining the thermal expansion of the framework along its principal axis, highlighting the importance of such analysis in low symmetry materials.
abstracts
Jarosites and related minerals are of great interest to a range of mineral processing and research applications. In some industrial settings jarosite formation is encouraged; for example to aid the removal of iron species from solutions in hydrometallurgical processes. In other environments such as bioleaching, jarosite formation can hinder the process by creating a kinetic barrier, in the form of a passivation layer, to the desired reaction. Jarosites are a major component of acidic soils and are present in significant amounts in acid mine drainage environments. There has been a recent resurgence in interest in jarosite minerals since their detection on Mars by the MER rover Opportunity. In this context, the presence of jarosite has been recognised as a likely indicator of the presence of water on Mars in the past. It is hoped that study of their formation mechanisms, stability and thermoelastic properties will provide insight into the environmental history of Mars as well as informing terrestrial industrial concerns. To this end we are engaged in a program to study jarosites and their formation and stability behaviour over a range of conditions. This contribution describes in situ powder diffraction experiments to determine the thermal expansion of a deuterated natrojarosite. Data were collected on the HRPD beamline at the ISIS spallation source where the natrojarosite sample was heated from 10-700K, and at the powder diffraction beamline at the Australian synchrotron where the sample was heated from 80-700K. Isothermal neutron and synchrotron datasets were refined simultaneously. Analysis of the lattice parameter variation with temperature shows that all cell edges increase smoothly to ~500 K where there is a discontinuity. This discontinuity represents the initially non-stoichiometric monoclinic jarosite converting to a stoichiometric, rhombohedral phase, shortly after which FeOHSO4 peaks become visible. Thermal expansion coefficients have been fitted from 10-470K and show that there is most variation in the monoclinic c-axis. This direction is normal to the layers of sulphate tetrahedra and iron octahedra within the jarosite structure and contains more flexible hydrogen bond linkages which more easily accommodate expansion than the more rigid polyhedra. Details of the combined neutron-synchrotron diffraction approach will also be discussed.
