Synthesis, Structural Characterisation and Physical Properties of Metal Oxychalcogenides

Abstract

Chapter 1 is a literature review on oxychalcogenide materials, with emphasis on structures similar to those discussed in later chapters of this thesis. These materials display a whole host of interesting properties and have garnered particular interest in recent years.

Chapter 2 describes the synthetic methods and characterisation techniques used to study the materials discussed in this thesis. It includes the theory behind powder diffraction techniques, Rietveld refinement, and a range of physical property measurement techniques.

Chapter 3 discusses the synthesis, structural characterization, and physical properties of the new transition metal oxyselenide Ce2O2ZnSe2. It adopts a ZrCuSiAs-related structure with Zn2+ cations in a new ordered arrangement within [ZnSe2]2– layers. The cell volume of the sample, and consequently the physical properties, can be controlled by subtle modification of the synthetic conditions. Ce2O2ZnSe2 is a semiconductor at all cell volumes with experimental optical band gaps of 2.2, 1.4, and 1.3 eV for high-, intermediate-, and low-cell volume samples, respectively. SQUID measurements show Ce2O2ZnSe2 to remain paramagnetic down to low temperature.

Chapter 4 reports 60 new compositions across the La2O2(Fe1–yZny)Se2, La2O2(Zn1–yMny)Se2, La2O2(Mn1–yCdy)Se2, Ce2O2(Fe1–yZny)Se2, Ce2O2(Zn1–yMny)Se2, Ce2O2(Mn1–yCdy)Se2, La2 zCezO2FeSe2, La2–zCezO2ZnSe2, La2–zCezO2MnSe2, and La2–zCezO2CdSe2 solid solutions. These series reveal that the transition metal arrangement in the Ln2O2MSe2 (Ln = La & Ce, M = Fe, Zn, Mn & Cd) compounds can be systematically controlled by either Ln or M substitution leading to an “infinitely adaptive” structural family.

Chapter 5 describes a new family of compounds containing both +1 and +2 transition metal ions in the La2O2Cu2–2xCdxSe2 family. It shows how Cu1+ and Cd2+ ions segregate into distinct fully occupied and half occupied checkerboard-like layers respectively, leading to complex long-range superstructures in the 3rd (stacking) dimension. To understand the structure and microstructure of these new materials a new methodology for studying low-probability stacking faults using a Rietveld-compatible supercell approach was developed and applied.

Chapter 6 investigates the solid solution La2–zSrzO2Cu0.5CdSe2. This takes the compound La2O2Cu0.5CdSe2 (x = 0.5 from Chapter 5, with a 1:1 ratio of Cu:Cd layers) and Sr2+ dopes on the La3+ site, where the solubility limit is shown to be ~15% (z = 0.3). At z = 0, ρ(300 K) = 1.5×104 Ω cm. After Sr doping, ρ(300 K) decreased significantly to 8.2×101 Ω cm at z = 0.05, reaching a minimum at ~6×10 1 Ω cm for z = 0.3. DFT calculations of La2O2CuCd0.5Se2 show that the valence band consists of Cu 3d and Se 4p states, hence electronic conduction should be confined to [Cu2Se2]2- layers.

Chapter 7 extends the work of Chapter 5 by discussing three more solid solutions analogous to La2O2Cu2–2xCdxSe2. These are La2O2Cu2–2xFexSe2, La2O2Cu2–2xZnxSe2 and La2O2Cu2 xMnxSe2. It also investigates the effect of systematically varying the +2 transition metal size in the solid solutions La2O2Cu(M0.5–yM’y)Se2 and La2O2Cu0.667(M0.667–yM’y)Se2 (M/M’ = Fe/Zn, Zn/Mn & Mn/Cd).

Chapter 8 will briefly summarise and link the work described in Chapters 3 to 7.