Synthesis and characterisation of high entropy lanthanide oxychalcogenides via the thermolysis of mixtures of molecular precursors

Abstract

High entropy (HE) materials present a fascinating and vast research space, though their synthesis remains a significant challenge in materials chemistry. The thermolysis of mixtures of dithiocarbamate-derived molecular precursors has been implemented in the past to produce higher sulfides but is an as yet unexplored route towards HE materials. In this thesis, the synthesis of novel HE lanthanide oxysulfides and oxysulfates are explored, produced via the solventless thermolysis of multiple dithiocarbamate precursors. The decomposition of the precursor mixtures produced oxysulfides in an argon atmosphere and oxysulfates in air. The resulting samples contained monophasic crystalline materials with a continuous distribution of the alloyed elements across millimetre and nanometre length scales. This was confirmed using powder X-ray diffraction (PXRD) with a combination of scanning and scanning transmission electron microscopies (SEM and STEM) with energy dispersive X-ray spectroscopy (EDX) mapping. The band gaps of all the bulk materials produced were estimated using UV-visible absorption spectroscopy and were found to have wide, direct band gaps between 3.7-3.9 eV for the HE oxysulfides and 4.0-4.1 eV for the HE oxysulfates. The same precursors were also decomposed in a mixture of high boiling point capping agents, in order to produce the analogous lanthanide oxysulfide nanoplates which were approximately 7 nm in the longest lateral dimension and either two or three unit cells thick (< 2.2 nm). This allowed for the direct experimental observation of quantum confinement between the two compositionally similar materials in a 170 nm blue shift of the optical absorption edge and a similar 40 nm shift in the photoluminescence peak. This experimental finding was supported by density functional theory (DFT) calculations and was the first reported observation of this effect in a HE material. Finally, the variable temperature magnetic susceptibility of a series of HE oxysulfides was also measured. Here, it was observed that the chemical disorder led to a suppression of magnetic order in the materials.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	David Binks (Supervisor) & David Lewis (Supervisor)