Design of new materials with crystal structure-related properties : The role of lone pair cations

Abstract: Non-centrosymmetry and low-dimensional arrangements, e.g. chains or layers, are very attractive structural characteristics of crystalline compounds since they are linked to some physical properties including nonlinear optical activity, ferroelectricity and magnetic frustration. The insertion of a lone-pair cation, equipped with a stereochemically active electron pair into a compound may increase its structural variety and induce the aforementioned characteristics.In this thesis some novel oxide and oxohalide compounds are described. They contain a transition metal and also a p-element lone-pair cation. Relevant synthesis details, crystal structure and physical properties of the compounds will be presented.Chemical systems containing iodate ions and either Cu2+ or Sc3+ have been explored in an effort to find new non-centrosymmetric compounds. The compounds contain also K+ due to its tendency to form highly coordinated asymmetric units. The new iodates are KCu(IO3)3 and the non-centrosymmetric compounds K3Sc(IO3)6 and KSc(IO3)3Cl.Only a few oxofluoride compounds that contain lone-pair cations have been reported in the literature, mainly due to synthetic difficulties. In this work new oxofluoride compounds containing the transition metals Cu2+, Co2+, or Sc3+ and the lone pair cations Bi3+ or Se4+ have been synthesized, where the transition metal octahedra form low-dimensional arrangements in form of chains or layers. The electronegative F ̶ anions behave like O2 ̶  and bridge in between different building blocks. The new oxofluorides are Cu2SeO3F2, CoBi2O2F4 and ScBi2O3F3. The first is a framework-like compound and the latter two are layered and belong to the Aurivillius family with one perovskite like layer. The inclusion of F made it possible to broaden this Aurivillius family to contain low-oxidation state transition metals. The magnetic properties of the new compounds containing Cu2+ and Co2+ were characterised and the Sc3+ containing non-centrosymmetric iodates were found to show non-linear optical properties.