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Title: Soft chemical control of layered oxychalcogenides
Author: Blandy, Jack
ISNI:       0000 0004 6500 6884
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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The structure, magnetic behaviour and chemistry of layered oxychalcogenides of composition A2MO2X2Ch2 (where A = Sr, Ba; M = Mn, Co, Ni, Cu, Zn; X = Cu, Ag and Ch = S, Se, Te) has been investigated by the synthesis of new compounds of this type and control of the properties of these compounds by oxidative deintercalation of Cu/Ag. I2 can be used to oxidatively deintercalate Cu from Sr2MnO2Cu1.5S2, forming Sr2MnO2Cu1.33S2, an incommensurately modulated compound, with a completely different Cu/vacancy ordering and antiferromagnetic ordering structure to the parent. This reaction is also probed in real-time, using in situ powder X-ray diffraction. Sr2MnO2Ag1.5Se2 was found to have an A-type magnetic ordering structure, similar to Sr2MnO2Cu1.5Se2. Sr2MnO2Cu1.8Te2 on the other hand with a lower Mn oxidation state shows only two-dimensional magnetic correlations, rather than long-range order. Extending the reaction with I2 to several Co-containing analogues revealed that ~ 25% Ag could be removed from Sr2CoO2Ag2Se2, sufficient to observe a change in magnetic behaviour, from antiferromagnetic to ferromagnetic. By contrast only ~11% Cu can be deintercalated from Sr2CoO2Cu2S2 and even less (~5%) from Sr2CoO2Cu2Se2. Neutron diffraction was used to examine the resultant changes in magnetic ordering. The novel compounds Sr2CuO2Cu2Se2 and Ba2CuO2-xCu2Se2 are related by substitution of the alkali-earth metal, but while Sr2CuO2Cu2Se2 is a stoichiometric compound with metal-like character, Ba2CuO2-xCu2Se2 is an oxygen-deficient semiconductor, with tuneable oxygen content. Unusual features are observed in the magnetic susceptibility measurements of Sr2NiO2Cu2Se2 that appear unrelated to this compound's long-range magnetic ordering, as probed by neutron diffraction. Furthermore, unusual peak splitting is observed in low-temperature powder X-ray diffraction patterns of this compound; this may plausibly be due to a photon-induced effect arising from the use of a high-energy beamline; although further measurements are required to examine this. Overall the work shows the flexibility and range of behaviour exhibited by a series of the transition metal oxide chalcogenides.
Supervisor: Clarke, Simon Sponsor: Engineering and Physical Sciences Research Council ; Diamond Light Source
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Chemistry ; Inorganic ; Magnetic order ; Oxychalcogenides ; Magnetism ; Solid-state chemistry ; Oxide chalcogenides