The oxidation states of Fe and Co in the complex perovskite-type oxide Ba 0.1 Sr 0.9 Co 0.8 Fe 0.2 O 3Àd were determined by means of X-ray absorption near edge structure (XANES) spectroscopy. Measurements performed in situ for 783 # T/K # 918 and 50 # pO 2 /Pa # 2 Â 10 4 reveal that the electrons generated by reduction are not equally distributed between the Fe and Co cations. A comparison with thermogravimetric measurements indicates a discrepancy in the amount of oxygen lost from the sample. This discrepancy is an indication that not only are the transition metal cations reduced but the oxygen anions are, too. A model explaining the observed behaviour is proposed.

The ammonolysis of beta-Ga(2)O(3) to alpha-GaN and the oxidation of alpha-GaN to beta-Ga(2)O(3) have been studied by means of in situ X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). In situ X-ray absorption measurements on polycrystalline powder particles on the gallium K-edge during both reactions give detailed information about the reaction kinetics. We were able to extract this kinetics by fitting linear combinations of beta-Ga(2)O(3) and alpha-GaN spectra only. The kinetics of the ammonolysis can be described well by an extended Johnson-Mehl-Avrami-Kolmogorow model, while the oxidation kinetics can be modelled by a shrinking core model. Investigations by means of TEM and SEM support the assumptions in both models. Our experimental results and the models are discussed in terms of the reaction energetics and the reaction mechanisms.

In situ XAS Nucleation Non-stoichiometry Amorphous oxide Insulator-metal transition a b s t r a c t Within the ternary system Ga-O-N we performed experimental and theoretical investigations on the thermodynamics, structure and kinetics of new stable and metastable compounds.We studied the ammonolysis of b-Ga 2 O 3 at elevated temperatures by means of ex situ X-ray diffraction, ex situ neutron diffraction, and in situ X-ray absorption spectroscopy (XAS). From total diffraction pattern refinement with the Rietveld method we analyzed the anionic occupancy factors and the lattice parameters of b-Ga 2 O 3 during the reaction. Within the detection limits of these methods, we can rule out the existence of a crystalline oxynitride phase that is not derived from wurtzite-type GaN. The nitrogen solubility in b-Ga 2 O 3 was found to be below the detection limit of about 2-3 at.% in the anionic sublattice. The kinetics of the ammonolysis of b-Ga 2 O 3 to a-GaN and of the oxidation of a-GaN to b-Ga 2 O 3 was studied by means of in situ X-ray absorption spectroscopy. In both cases the reaction kinetics could be described well by fitting linear combinations of b-Ga 2 O 3 and a-GaN spectra only, excluding that other crystalline or amorphous phases appear during these reactions. The kinetics of the ammonolysis can be described well by an extended Johnson-Mehl-Avrami-Kolmogorow model with nucleation and growth of GaN nuclei, while the oxidation kinetics can be modeled by a shrinking core model where Ga 2 O 3 grows as a layer. Investigations by means of TEM and SEM support the assumptions in both models.To investigate the structure and energetics of spinel-type gallium oxynitrides (g-galons) we performed first-principles calculations using density-functional theory. In addition to the ideal cubic g-Ga 3 O 3 N we studied gallium deficient g-galons within the Constant-Anion-Model.In highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO 1.2 we found at a temperature around 670 K an insulator-metal transition, with a conductivity jump of seven orders of magnitude. We demonstrate through experimental studies and density-functional theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric b-Ga 2 O 3 within the metastable oxide matrix. By doping with nitrogen the critical temperature and the conductivity in the highly conducting state can be tuned.

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GaN Ammonolysis Oxynitrides Nitrogen solubility a b s t r a c tWe investigated the ammonolysis of b-Ga 2 O 3 at elevated temperatures by means of ex situ X-ray diffraction, ex situ neutron diffraction and in situ X-ray absorption spectroscopy. Within the detection limits of these methods, we can rule out the existence of a crystalline or amorphous oxynitride phase that is not derived from wurtzite-type GaN. No evidence for a b-Ga 2 O 3 related oxynitride phase was found, and the nitrogen solubility in b-Ga 2 O 3 was found to be below the detection limit of about 2-3 at% in the anionic sublattice. These findings were obtained by monitoring the anionic occupancy factors and the lattice parameters of the b-Ga 2 O 3 phase obtained from total diffraction pattern refinement with the Rietveld method and by linear combination fitting of the X-ray absorption spectra that were recorded during the ammonolysis.

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