Synthesis and characterisation of S = 1/2 kagome magnets in metal-organic frameworks

Dolling, Tristan Niles ORCID: 0009-0007-9324-1485 (2025). Synthesis and characterisation of S = 1/2 kagome magnets in metal-organic frameworks. University of Birmingham. Ph.D.

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Abstract

Magnetic frustration is a phenomenon that may occur in materials when the arrangement of magnetic atoms in a structure results in competing magnetic exchange interactions. One example of this is the kagome arrangement, featuring corner sharing triangles of magnetic atoms, this motif is expected to generate magnetic frustration in cases where antiferromagnetic order is present. In this thesis, several magnetic kagome containing metal-organic frameworks were investigated. This diverse class of materials, comprised of magnetic metal nodes and organic ligands, provides the opportunity to examine the bulk magnetic behaviours that arise due to the use of distinct geometries, such as the kagome, which is anticipated to host interesting magnetic characteristics at low temperatures. In our initial investigation, we focus on a single member of the Cu\(_{3}\)(CO\(_{3}\))\(_{2}\)(x)\(_{3}\)·2ClO\(_{4}\) family of metalorganic frameworks, where x represents a ditopic ligand such as bis(3-pyridyl)acetylene. This metal-organic framework features Cu\(^{2+}\), CO\(_{3}\)\(^{2−}\) kagome layers which are coordinated by the ditopic ligands to form a three-dimensional network. Using X-ray and neutron diffraction techniques, along with magnetic susceptibility measurements, we were able to definitively characterise the crystal structure and magnetic properties of this MOF. The magnetic properties we describe were consistent with those observed in other compounds belonging to the same Cu\(_{3}\)(CO\(_{3}\))\(_{2}\)(x)\(_{3}\)·2ClO\(_{4}\) family, where ferromagnetic and antiferromagnetic order coexist.

In our second investigation, we study the impact of eliminating covalent bonding between the kagome layers. To do this, we explore two additional systems with the formula Cu\(_{3}\)(CO\(_{3}\))\(_{2}\)(x)\(_{6}\)·2ClO\(_{4}\), where x represents a monodentate ligand such as methylpyridine or 2,4-bipyridine. These monodentate ligands give rise to a non-covalent, stacked arrangement of Cu\(^{2+}\), CO\(_{3}\)\(^{2−}\) kagome layers, which were expected to suppress or eliminate magnetic exchange interactions between nearest neighbour kagome layers, forming a magnetic quasi-two-dimensional kagome structure. Using X-ray/neutron diffraction, heat capacity, and magnetic susceptibility measurements, we were able to confirm the existence of ferromagnetic exchange interactions within the kagome layers, with no evidence for antiferromagnetic exchange interactions between layers. Furthermore, we extend our investigation by exfoliating these three-dimensional materials to the monolayer limit, tracking success using atomic force microscopy. Our observations reveal compelling evidence for the successful exfoliation to a monolayer; however, further examination is required for the confirmation of crystallinity and magnetic activity.

In our final investigation, we explore the impact of altering intra-plane exchange pathways on magnetic exchange interactions within a kagome lattice by synthesising a two-dimensional MOF with the formula Cu\(_{3}\)(C\(_{6}\)O\(_{6}\))\(_{2}\). This material features no inter-plane coordination and employs a distinct superexchange pathway anticipated to accommodate antiferromagnetic exchange interactions. By conducting high-resolution X-ray powder diffraction measurements, we were able to dispel contradictory findings from the literature, with our data suggesting a \({C2/m}\) space group assignment. Furthermore, by examining heat capacity, magnetic susceptibility, and muon spin relaxation data, we observe no sign of a well-defined long-range ordered ground state. This suggested that this system may host an exotic magnetic ground state at low temperature, one such as the elusive quantum spin liquid state.

The overall conclusion of this research is that metal-organic framework chemistry provides an excellent experimental route to characterising physical examples of theoretically significant magnetic models.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):