Matthias Knoll «  Rosiana Aguiar »  Jochen Oelke
Rosiana Aguiar
Synthesis, Properties and Applications of AB(O,N)3 Oxynitride Perovskites
Supervisor: Prof. Dr. Armin Reller [Solid state chemistry]
Date of oral examination: 12/05/2008
Perovskites are a highly interesting class of materials. Their physical and chemical properties can be tailored by varying the cationic/anionic stoichiometries. While cationic substitutions have been intensively studied, substitutions in the anionic sublattice are by far less well examined. This work describes the different synthesis routes to prepare oxynitride perovskites of the general composition AB(O,N)3 with A = Ca, Ba, Sr, La, Nd and B = Ti, Nb, Ta, as well as their properties and possible applications. The oxynitrides were either obtained directly from mixtures of binary oxides/carbonates or from complex perovskite-related oxides of composition ABO3.5 or ABO4. The oxide precursors were prepared as polycrystalline samples by different synthesis techniques such as solid-state reaction, Pechini method, spray pyrolysis and polyol assisted coprecipitation. Thin films of these oxides were deposited by spin coating and pulsed laser ablation. Single crystals have been obtained using a floating zone furnace with radiation heating. The corresponding oxynitrides were synthesized by reaction with ammonia gas at high temperatures, commonly denoted as ammonolysis, in conventional tube furnaces or by a microwave induced ammonia plasma. The compositions, crystal structures and the physical properties of the samples were analyzed by a variety of different techniques such as x-ray and neutron diffraction, atomic force microscopy, transmission and scanning electron microscopy, O/N hot gas extraction, secondary ion mass spectrometry, thermogravimetry and mass spectrometry, UV-vis spectroscopy, etc. The introduction of nitrogen in the oxide lattice results in a reduction of the band gap, because N3- is less electronegative than O2-. As a consequence the oxynitride compounds start to absorb light in the visible range. The possibility to use the samples as non-toxic pigments and photocatalysts have been studied. The thermal stability of the oxynitride samples under different oxygen concentrations was also analyzed. Finally, the physical properties of the oxynitride single crystalline layers prepared by ammonolysis of oxide crystal slices were investigated. The oxynitride layers presented electrical conductivity ca. 5 orders of magnitude higher than the corresponding oxide slices. It was verified that the oxynitride perovskites could be in the near future used as environmental friendly pigments, substituting some toxic dyes that contain heavy metals. The samples also showed significant photocatalytic efficiency for the decomposition of organic molecules and for evolution of H2 and O2 from aqueous solutions.