Structure, Synthesis and Properties of d-block Metal Sulfides and Oxychalcogenides

Abstract

Non-oxide and mixed-anion materials are able to exhibit a variety of properties that are out of reach for single-anion oxides. Most oxides have large bandgaps, making them photoresponsive within the UV region, which makes up only a small portion of the solar energy that reaches the Earth’s surface. In the search for photocatalysts which can make use of a wider range of solar energy, the narrower bandgaps of both sulfides and oxychalcogenides can be exploited. Evaluating the crystal and electronic structures of these materials can help to find photocatalysts which are suitable for the water-splitting reaction, giving the ability to generate green hydrogen from water, helping accelerate the transition to greener energy sources.

Additionally, mixed-anion materials containing magnetic cations can give rise to complex magnetic structures, with the magnetic ions able to communicate through multiple anion species. Specific arrangements of the magnetic cations can give rise to geometrical frustration which then encourages non-collinear magnetic order. Non-collinear antiferromagnets could be used in future spintronic devices, enabling more stable sensors and memory devices to be realised.

This thesis uses a combined approach of computational and experimental techniques. First principles calculations are employed to identify the stability and synthesisability of novel sulfides (LaMS2, where M = Cu, Ag) and oxychalcogenides (La2O2MQ2, where M = Cu, Zn, Ag, Sn and Q = S, Se). These are initially investigated as potential photocatalysts for the solar-driven water-splitting reaction and are targeted using both traditional and lower-temperature synthesis methods. The resultant products are characterised, and density functional theory is used to give insights into the experimental observations.
This research also investigates the magnetic structures of BaFe12Se7O6, which exhibits a non-collinear magnetic structure at lower temperatures. To better understand these magnetic phase transitions, the exchange interactions between the magnetic ions are evaluated using density functional theory.