High-Pressure Synthesis, Crystal Structure and Physical Properties of Gallium Oxonitride

This thesis is concerned with the synthesis and properties of the high-pressure, high-temperature (HP/HT) phase of gallium oxonitride, which has a spinel-type structure. Since this material is an analogue of γ-alon (spinel-aluminium oxonitride), which is known to be an important material with a wide range of applications, the spinelstructured gallium oxonitride is an attractive material with the potential for tailoring its composition and tuning its electronic properties, i.e. for optoelectronic applications. The research reported in this thesis is focused on two main aspects:firstly the HP/HT synthesis of the spinel phase using different starting materials and employing different high-pressure devices, and secondly a detailed characterisation of the HP material. For the synthesis of the spinel-structured gallium oxonitride, different starting materials have been used. For a systematic investigation of the pressure-temperature relation in the Ga-O-N system, different molar ratios of the w-GaN and β-Ga2O3 were tested for the HP/HT syntheses. Large-volume presses provided h higher amounts of samples for further investigations of the spinel phase. Diamond anvil cells (DACs)loaded with same powder mixtures provided valuable insights for the formation of the spinel phase. A different starting material also employed for the HP/HT syntheses was a precursorderived gallium oxonitride (GaON) ceramic. In order to obtain the GaON ceramic,gallium tris(t-butoxide)-dimethylamine adduct, Ga(OtBu)3.HNMe2, was synthesised and heat treated in an ammonia flow. The obtained ceramics were x-ray amorphous and stable up to around 1000 K. The HP/HT experiments using the (GaON) ceramics revealed a phase transition from the amorphous phase to a spinel-structured gallium oxonitride at 0.7 GPa and 1600 K. The use of the single source precursor GaON resulted in the formation of the spinel phase at much lower pressures than in the syntheses performed using different mixtures of GaN and Ga2O3. Hence, a piston cylinder press has been used to provide larger amounts of the spinel phase from the (GaON) ceramic for different ex situ investigations. A third starting material for the syntheses of the spinel gallium oxonitride was metastable cubic γ-Ga2O3. When loaded in a DAC with nitrogen gas under pressure,the oxide reacted with nitrogen, forming the spinel-type gallium oxonitride at pressures of 4.2 GPa and temperatures between 1500 and 1800 K. The second part of this thesis deals with the structural characterisation of the spinel gallium oxonitride and an investigation of its properties. A single crystal structure investigation of the HP/HT phase of gallium oxonitride has revealed a spinel structure within the space group Fd3m (No. 227) with the lattice parameter a0 = 8.2782 Å. Electron Energy-Loss Spectroscopy (EELS) has enabled the relative amounts of nitrogen and oxygen to be quantified for structural refinement. The structure analyses revealed that for the spinel phase, the composition is 2.7