Soft chemical control of layered oxychalcogenides

Thesis

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 Sr₂MnO₂Cu_1.5S₂, forming Sr₂MnO₂Cu_1.33S₂, 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.

Sr₂MnO₂Ag_1.5Se₂ was found to have an A-type magnetic ordering structure, similar to Sr₂MnO₂Cu_1.5Se₂. Sr₂MnO₂Cu_1.8Te₂ 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 Ba₂CuO_2-xCu₂Se₂ are related by substitution of the alkali-earth metal, but while Sr₂CuO₂Cu₂Se₂ is a stoichiometric compound with metal-like character, Ba₂CuO_2-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.

Authors: Blandy, J

• DOI: 10.5287/ora-rvpannzvb
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Oxychalcogenides; Magnetic order; Magnetism; Solid-state chemistry; Oxide chalcogenides
• Subjects: Chemistry, Inorganic