The structure, magnetic behaviour and chemistry of layered oxychalcogenides of composition A<sub>2</sub>MO<sub>2</sub>X<sub>2</sub>Ch<sub>2</sub> (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. I<sub>2</sub> can be used to oxidatively deintercalate Cu from Sr<sub>2</sub>MnO<sub>2</sub>Cu<sub>1.5</sub>S<sub>2</sub>, forming Sr<sub>2</sub>MnO<sub>2</sub>Cu<sub>1.33</sub>S<sub>2</sub>, 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. Sr<sub>2</sub>MnO<sub>2</sub>Ag<sub>1.5</sub>Se<sub>2</sub> was found to have an A-type magnetic ordering structure, similar to Sr<sub>2</sub>MnO<sub>2</sub>Cu<sub>1.5</sub>Se<sub>2</sub>. Sr<sub>2</sub>MnO<sub>2</sub>Cu<sub>1.8</sub>Te<sub>2</sub> 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 I<sub>2</sub> to several Co-containing analogues revealed that ~ 25% Ag could be removed from Sr<sub>2</sub>CoO<sub>2</sub>Ag<sub>2</sub>Se<sub>2</sub>, sufficient to observe a change in magnetic behaviour, from antiferromagnetic to ferromagnetic. By contrast only ~11% Cu can be deintercalated from Sr<sub>2</sub>CoO<sub>2</sub>Cu<sub>2</sub>S<sub>2</sub> and even less (~5%) from Sr<sub>2</sub>CoO<sub>2</sub>Cu<sub>2</sub>Se<sub>2</sub>. Neutron diffraction was used to examine the resultant changes in magnetic ordering. The novel compounds Sr<sub>2</sub>CuO<sub>2</sub>Cu<sub>2</sub>Se<sub>2</sub> and Ba<sub>2</sub>CuO<sub>2-x</sub>Cu<sub>2</sub>Se<sub>2</sub> are related by substitution of the alkali-earth metal, but while Sr<sub>2</sub>CuO<sub>2</sub>Cu<sub>2</sub>Se<sub>2</sub> is a stoichiometric compound with metal-like character, Ba<sub>2</sub>CuO<sub>2-x</sub>Cu<sub>2</sub>Se<sub>2</sub> is an oxygen-deficient semiconductor, with tuneable oxygen content. Unusual features are observed in the magnetic susceptibility measurements of Sr<sub>2</sub>NiO<sub>2</sub>Cu<sub>2</sub>Se<sub>2</sub> 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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:735911 |
| Date | January 2017 |
| Creators | Blandy, Jack |
| Contributors | Clarke, Simon |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://ora.ox.ac.uk/objects/uuid:0fcc2604-0e0d-4206-a0aa-3fdcc611e357 |
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