Structures, Properties and Chemistry of Layered Oxide Chalcogenides

The project will include investigating the structures and properties of layered oxide chalcogenides and tuning their magnetic and electronic properties using chemical substitution. Oxide chalcogenides are an extremely versatile class of compound, owing to the anion segregation observed due to the chemical requirements of the smaller oxide anion and larger, more polarisable chalcogenide anions, which leads to these materials having layered structures. These layers can then possibly host cations such as Li+, which can then be cycled to test battery cathode suitability which aligns well within the research goals of EPSRC and the other government-funded bodies such as the Faraday Institution). Other applications of oxide chalcogenides include semi-conductors, transistors and also as a possible source of renewable energy, for example, in solar panels.
This project will produce multiple novel compounds based on compounds in the literature. As mentioned previously, oxide chalcogenides are compositionally flexible, this means that new compounds can be derived from already known structures via substitution or doping. These substitutions can lead to fine tuning of the electrical and magnetic properties. This is known in, for example, NaFeAs, whereby doping with another transition metal such as Co or Ni can induce superconducting behaviour.
Although the synthetic methodology is relatively straightforward, involving weighing out reactants and grinding together in an agate mortar and pestle before sealing in an evacuated silica ampoule; the characterisation of the compounds that will be produced during this project is certainly not trivial. A wide range of techniques will be employed to complement the in-house X-ray powder diffraction data as well as the magnetisation as measured using a SQUID magnetometer. High quality X-ray data for detailed characterisation will be obtained at the Diamond Light source, and neutron diffraction at the ISIS Facility to probe magnetic ordering within the compounds; the neutron source at the ILL (Grenoble, France) will also be used for this. Some samples may not be able to be synthesised at ambient pressures and these will be carried out with help from Element 6 (Harwell) or via a new Core-to-Core collaboration with JSPS (Japan). Other techniques that will be less frequently used include (but not limited to), electron diffraction (Antwerp), Mössbauer spectroscopy (Sheffield Hallam), electrical resistivity and single crystal growth.
The initial targets of the project are from the solid solution of Sr2NiO2Cu2S2 (low spin) and Sr2NiO2Cu2Se2 (high spin), which both lie close to the spin boundary as supported by density functional theory calculations. Preliminary results indicate that the spin-state crossover has already occurred by the time the series has reached Sr2NiO2Cu2SeS and so further investigation is needed into the already synthesised more Se rich compounds, as well as the synthesis of the more S rich compounds via a eutectic halide flux to confirm the findings. Furthermore, attempts to dope the Se analogue with calcium to increase the ligand field at Ni2+ to induce the spin-state crossover seem promising although results would suggest that all the intended Ca2+ is not being introduced into the structure, this could potentially be overcome via a high-pressure synthesis. Further investigations will include analogues containing other transition metals. This project falls within the EPSRC 'Physical Sciences' research area.