Structural Analysis of Functional Coordination Frameworks

This Thesis seeks to advance the understanding of how the breadth of physical properties displayed by coordination frameworks arise. Structural analysis informs us of the geometry of the chemical structures on the atomic scale, providing insight into the structural origins of their macroscopic material properties. Analysis of the structure and dynamics of a system can also inform on the material's properties and is done by combining analysis using X-rays and neutrons to provide a fuller understanding of these hybrid systems. Coordination frameworks combine organic species with metals to form diverse structure types, giving various properties and consequent applications. These include dielectric properties discussed in Chapter 3, mechanical properties discussed in Chapter 4, and magnetic properties discussed in Chapter 5.

Ferroelectric frameworks have recently been shown to have relaxor ferroelectric properties. Relaxors exhibit a broad temperature and frequency dependence, giving improved tolerances to operating these as real-world devices. Most existing relaxor materials exhibit their properties due to chemical inhomogeneity, e.g. disorder of A and B site cations in oxide perovskite relaxors. Chapter 3 explores the ferroelectric frameworks [NH3NH2]Mg(CO2)3 and [NH4]M(CO2)3 where M = Mn & Zn, that have been reported to show relaxor-like dielectric spectra, seeking the structural and dynamic origins of this relaxor behaviour. There is a focus on the A-site cations of these structurally similar frameworks, as each shows very different dielectric properties with evidence from single crystal neutron diffraction, quasielastic neutron scattering and solid-state NMR that the relaxor-like properties likely stem from rotational disorder, although the nature of this varies between the two families of compounds.

Coordination frameworks can also exhibit unusual mechanical properties and include responses to temperature and pressure. It has been shown that some coordination frameworks will exhibit unusual behaviour of contracting on heating or expanding under pressure in specific directions. This study explores the response of two [A]Er(HCO2)2(C2O4) (A = (NH2)3C or (CH3)2NH2) phases, along with a related [A]Er(HCO2)(C2O4)1.5 phase to pressure using synchrotron single crystal X-ray diffraction. This reveals that [(NH2)3C]Er(HCO2)2(C2O4) is the first hybrid perovskite to exhibit negative linear compressibility, in which a material expands along one axis under pressure from ambient pressure and across a significant pressure range. The properties of these systems are studied further in Chapter 4.

The magnetic properties of lanthanide-based coordination frameworks have applications as replacements for liquid helium in ultracold refrigeration units. This is particularly important as helium is an overlooked non-renewable resource currently utilised in cooling scientific and medical instruments such as MRI machines. By understanding the structural features that contribute to the magnetocaloric effect of coordination frameworks, new magnetocaloric materials can be produced as an alternative method of cooling these medical and scientific devices. Chapter 5 explores the effect of doping diamagnetic yttrium into Dy(OH)CO3, which is a promising magnetocaloric. This study finds that alleviating the frustration by doping likely results in increased antiferromagnetic coupling and decreased magnetocaloric performance of this system.