Synthesis and structural characterisation of layered oxide halides

Needs, Richard Leslie (1997) Synthesis and structural characterisation of layered oxide halides. University of Southampton, Doctoral Thesis.

Abstract

A wide range of layered complex oxide fluorides, oxide chlorides and oxide bromides of indium, scandium, titanium, iron and ionbium have been synthesised. Syntheses were performed by conventional solid state methods or using a cold seal hydrothermal bomb. These new materials have been investigated and structurally characterised by an appropriate combination of the following techniques; Rietveld refinement of powder x-ray and neutron diffraction data, ¹⁹F MAS NMR spectroscopy, ⁵⁷Fe Mossbauer spectroscopy, EXAFS. Oxide fluoride ordering was determined via a consideration of both bond valence calculations and ¹⁹F MAS NMR data.

The barium indium oxide halide series Ba₂InO₃X (X=F, Cl, Br) are all isostructural and were refined using the novel variant of the K₂NiF₄ structure. The proposed structure is tetragonal with the space group P4/nmm and contains fully ordered, segregated, oxide and halide anions. The unit cell consists of layers of corner sharing InO₅ square based pyramids separated by BaX rock salt layers. The oxide fluoride member of the Ba₃In₂O₅X₂ (X=Cl, Br) Ruddlesden-Popper series has been successfully synthesised and shown to have a structure similar to that of the oxide chloride and oxide bromide. Attempts to confirm the presence and location of the fluoride anions, in both Ba₂InO₃F and Ba₃In₂O₅F₂, by the analysis of EXAFS data were inconclusive, but the fully ordered model has been corroborated by both ¹⁹F MAS NMR spectroscopy and bond valence considerations. Barium may be partially substituted by strontium in the oxide chloride Ba₂InO₃Cl, yielding BaSrInO₃Cl, where analysis EXAFS data has shown the alkaline earth cations to be disordered. A compound of nominal stoichiometry Ba₄In₂O₈Cl₂ was also prepared and found to adopt a Ruddlesden Popper type Sr₄Ti₃O₁₀ structure, with apical chloride anions. The actual stoichiometry was determined as near Ba₄In₂SnO_8.4Cl_1.2.

On complex scandate, analogous to the indium oxide halides, has been successfully synthesised as a pure phase, Ba₂ScO₃F. The structure has been refined using the T type K₂NiF₄ model, as opposed to the primitive distortion observed in the indate. Analysis of ¹⁹F MAS NMR data and bond valence calculations suggest that oxide and fluoride anions share disordered apical sites about the scandium. A perovskite, BaScO₂F, has been produced and structurally characterised as an ideal cubic perovskite with oxide and fluoride distributed over all anion sites. This is the first example of a perovskite with stoichiometry ABO₂F. This unexpected difference in the structures of Ba₂ScO₂F and Ba₂InO₃F has been discussed in terms of their electronegativity and preferred bonding requirements.

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