Spin-crossover is a high-spin-to-low-spin transition at a transition metal centre, induced by temperature or pressure. It is common in iron chemistry, but also occurs in some compounds of other common metals. Spin-crossover switching changes the volume, hardness and magnetic properties of the sample, and may also affect its colour, electrical resistance or other properties depending on the material. These properties have been harnessed in materials applications such as thermochromic inks; stimuli-responsive polymers; and solid state refrigeration.
Different materials can undergo spin-crossover abruptly or gradually with temperature, and with or without thermal hysteresis. This is governed by how strongly the individual switching centres in the material interact with each other, which in turn relates to its crystal packing. Designing a spin-crossover material with bespoke properties for a specific application is a challenging problem, which hinges on the structure, dynamics and cohesive forces within a crystal lattice.
This project will address part of this question, by correlating the switching characteristics of spin-crossover materials with their internal lattice vibrations. The student will use Leeds’ world-leading facilities for terahertz spectroscopy (that is, low energy vibrational spectroscopy). THz data will be combined with IR and Raman spectroscopy measurements, to image the vibrational landscape of selected materials and how it changes during the spin-crossover event.
A particular focus of the work is to identify collective lattice vibrations correlating with abrupt spin-crossover switching in these materials. This information will then be used to rationally design new materials to enhance the appropriate lattice dynamics and increase the cooperativity of their switching properties, to confirm the validity of that structure:function relationship.
As part of these investigations, we will study why replacing the hydrogen atoms with deuterium atoms in spin-crossover crystals changes their switching temperature. That is a 40-year old conundrum, which was first noted in the 1980s but has never been explained in any detail. Since deuterating a molecule changes its vibrations but doesn't affect its electronic character, that observation must have a vibrational explanation. Thus, addressing this question is also relevant to the wider goals of the project.
This project will involve organic and inorganic synthesis; crystallography and powder diffraction; solid state magnetic measurements; infrared, Raman and THz vibrational spectroscopy; and other physical techniques as appropriate. There will be opportunities to perform X-ray crystallography at the Diamond synchrotron.
The projected start date is October 1st, although a later start date may also be possible.