Pressure-Tuning Interactions in Molecule-Based Magnets

In optimizing the properties of functional materials it is essential to understand in detail how structure influences properties. Identification of the most important structural parameters is time-consuming and usually investigated by preparing many different chemical modifications of a material, determining their crystal structures, measuring their physical properties and then looking for structure-property correlations. It is also necessary to assume that the chemical modifications have no influence other than to distort the structure, which is often not the case.
High pressure offers a way around these difficulties. Pressure can be used to distort a material without the need for chemical modification. Both crystal structures and physical property measurements can be conducted at high pressure, so that the properties of the same material can be studied in different states of distortion, providing the most direct way to study correlations between structure and properties.
In this proposal we focus on structure-property relationships in molecule-based magnets connected into extended chains, networks or frameworks using a combination of high pressure crystallography, magnetic measurements, spectroscopy and simulation which will exploit the UK's unique capabilities in extreme conditions research. Extended materials are of great interest because a small distortion at one site is propagated throughout the material by the strong chemical links between the magnetic centres, making the magnetic properties very sensitive to structural changes. We will design and build new instruments for magnetic susceptibility and diffraction measurements at high pressure and low temperature and we will exploit these new instruments and methodology to study two important classes of magnetic material.
1-D magnetic materials represent a fertile playground for discovering and understanding exotic physical phenomena. The magnetic behaviour of Single-Chain Magnets (SCMs) is fundamentally governed by the magnitude of nearest neighbour exchange interactions (intra-chain exchange), the extent of inter-chain interactions, and Ising-like anisotropy - all of which are sensitive to pressure. We have already shown that these parameters can be pressure-tuned in Single-Molecule Magnets (SMMs) and the same should be true for SCMs In 3-D frameworks magnetism can be combined with porosity, so that inclusion of different guest molecules provides another means for controlling magnetic properties. Prussian Blue Analogues consist of different metal cations linked by cyanide anions, while metal carboxylates build diamond-like frameworks. In both cases guest molecules influence magnetic ordering temperatures. Some metal-organic frameworks show spin-crossover behaviour, where different electronic configurations of the metal ions are stable under different conditions. The transition from one form to another is influenced by guest molecules which occupy the pores of the framework. High pressure will enable us to control the structure of the framework itself, the interactions between the host and the guest, and the number of guest molecules incorporated into the pores, providing a quantitative link between host-guest interactions and magnetism.

The proposed programme is highly interdisciplinary in its scope, encompassing synthetic chemistry through to quantum physics and engineering, with a focus on the technically highly demanding area of extreme conditions. It provides an outstanding opportunity for the three researchers employed who will benefit from training in the most up-to-date scientific methodologies, publishing their work in the highest impact journals and presenting their work at an ambitiously broad range of conferences. Their skills will be broadened by proposal writing, (e.g. beamtime applications) and taking part in public engagement activities (see below). They will receive training outside the academic environment in the form of industrial internships which will provide valuable experience of working in the commercial sector. We have an outstanding record in training, and this has been recognised in the professional success of staff previously employed on our grants, including prizes and fellowships. The training the research staff will receive will make them extremely valuable for the UK economy, skilled in the most up-to-date research techniques, problem solving, writing for both publications and applications, presentation of work to specialists and non-specialists, trained in leadership and entrepreneurship with experience in academia, at central facilities and in the commercial sector.
Though extreme conditions research is most actively pursued in academia, our aim in this programme is to demonstrate its power in defining structure-property relationships. Providing a mechanism for determining how a structure should be tuned to optimize a particular property will find direct applications in industries that make use of materials science (electrical conductors, pressure-sensitive materials for security applications, catalytic applications of porous materials). The engineering expertise and the skills in design for extreme conditions are also of value to industries such as aerospace, renewable energy and advanced machining. Pressure is a more powerful thermodynamic variable than temperature and its potential for direct application in manufacturing processes is huge. It is already extensively used in materials synthesis and food processing, but we envisage that application in fine chemicals production and catalysis are realizable in the 10-15 year time-scale. This is the driver for fundamental extreme conditions research in the Chemical Sciences, and this growing interest is served by private-sector manufacturers of research instrumentation and software, several of whom are involved in our programme as hosts for internships. For example, we are working with the Cambridge Crystallographic Data Centre on methods, inspired by our work in high pressure, for the calculation of intermolecular interaction energies and visualisation of voids.
Our experience is that generation of very high pressures and the effect these have on materials rapidly attracts the interest of participants at public engagement activities. We regularly take on high-school students and undergraduates in summer projects based on our research and many of these have gone on to take science degrees or PhDs. We exploit the broad range of public engagement activities currently available in the three centres involved in this proposal to provide training in the ways to optimise presentation of complex material to the public to achieve greatest impact, a skill which will also prove extremely valuable in any profession.