Synthesis and Characterisation of Complex Oxychalcogenides

Mixed-anion compounds like oxychalcogenides, where there are both oxide anions and chalcogen anions (S-Te) create quite a rare class of materials which is gaining increasing attention. The anions have different sizes and chemical requirements which promise ordering of the oxide crystallographically, whereby their structures can be described as a mixture of 2-D structural building blocks (e.g. perovskite, fluorite, or rock-salt) stacked on top of each other along a given

direction, thus giving a layered structure. These results in interesting electrical, optical and physical properties. This thesis reports a study of oxychalcogenide compounds with general formula Ln2O2Fe2OQ2 (Ln = lanthanide, Q = chalcogenides) and LnMOCh2 (Ln = Lanthanides, M = cation and Ch = chalcogenides). These compounds offer big opportunities for chemical substitution and doping within and between the layers, hence it is possible to tune the layer distances and in turn, tune the properties of these solids. The crystal structures and properties of all materials were investigated using several in-house techniques and those at central facilities. Electron doping La2O2Fe2OSe2 with Co and Ni ions onto the Fe2+ site gave limited solid solutions and semiconductor materials. Co-doping increased the transition temperature of La2O2Fe2OSe2 indicating that the distance between the Fe2O layers is reduced. Variable temperature neutron powder diffraction data of La2O2Fe2OS2 showed 2-D short range order while neutron powder diffraction data in applied magnetic field of Pr2O2Fe2OSe2 showed a subtle orthorhombic distortion around 23 K. Additional magnetic Bragg reflections, seen in both compounds, are consistent with a 2-k magnetic structure with a 2a x 2a x 2c unit cell. Moreover, both compounds showed stacking faults in the magnetic ordering on the Fe2+ sublattice

which are absent in Mn2O or Co2O systems, implying that they are exclusively to the Fe2O layers and could be sensitive to the distance between Fe2O layers. Isovalent doping LaGaOS2 with Nd and Ce ions onto the La site gave limited solid solutions. From the diffuse reflectance spectroscopy data, Nd-doping and Ce-doping the parent compound gave no significant change in optical band gaps and

the colour change observed in colour in Nd-doped samples are presumably due to f-f transitions. This work suggests that LaGaOS2 has a quite limited compositional flexibility in its structure and that it is not easy to modify its electronic structure using chemical pressure. Electron doping BiCuOSe with Ce (onto the Bi3+ site) and F ions (onto the O2- site) gave limited solid solutions. BiCuOSe had some Cu vacancies. The Ce-doped samples contained Ce4+ ions. Electron doping BiCuOSe with F gave metallic compounds, while doping BiCuOSe with Ce gave compounds with both semiconducting and metallic behaviour. The semiconducting behaviour may be due to impurities.