Quantenchemische Untersuchungen metastabiler Oxide und Oxidnitride der Chrom- und Vanadiumgruppe

摘要

In this work an investigation of different metastable oxides and oxynitrides of the chromium and vanadium group employing density-functional theory is presented. The herein presented calculations deliver additional information in form of extended characterization of previously synthesized compounds such as δ-TaON, that are hard to obtain via experimental work. Furthermore, the calculations also present possible synthesis routes to new compounds, that are verified through experimental work, such as the high pressure modification of MoO2, or suggest possible metastable polymorphs of existing compounds. The compound TaON is known to form a few different polymorphs such as the recently synthesized δ-TaON with Anatasstructure. The most likely anionic ordering of δ-TaON and its dynamic stability could be verified through quantumchemical calculations. It is roughly 15.6 kJ/mol less stable than the thermodynamically stable Baddeleyitemodification and only 6.4 kJ/mol less stable than the metastable γ-TaON. Thermodynamic calculations reveal an instabillity compared to the other phases especially at low temperatures, the observed phase transition is therefore essentially controlled through kinetics. This modification can furthermore be classified as metastable. The dioxides of molybdenum and tungsten both crystallize in a distorted rutile-type, also known as VO2-type. Tungstendioxide is also known to form a high pressure modification in the [hp-WO2]-type that forms at about 5 GPa. A further possible high pressure polymorph could be identified for both dioxides: the orthorhommbic [ortho-TiO2]-type forms at about 28 GPa for MoO2 and 38 GPa for WO2. Three possible polymorphs could be identified for both dioxides at ambient pressure: [mod-TiO2], [SnO2(II)] und [α-PbO2]. The [mod-TiO2]-structure shows strong similarities to the thermodynamically stable [VO2]-structure and has possibly been synthesized for MoO2, but falsely characterised as a rutile-structure. Calculations on the sesquioxides of molybdenum and tungsten revealed a slightly modified corundum-structure as the most stable crystal structure for both compounds. A possible high pressure synthesis via the metal and the dioxide is hampered by the phase transition of the dioxides at 2 / 5 GPa, as the synthesis runs via the respective high pressure polymorphs. This results in a synthesis pressure of about 60 GPa for W2O3 and more than 100 GPa for Mo2O3. Both pressures are experimentally very hard to achieve, a synthesis of W2O3 seems to be more likely. Measurements on compounds with the general composition Mo2(O,N,□)5 suggest the [VNb9O24.9]-structure for the hypothetical phase Mo2O5. Quantum chemical calculations confirmed this structure as the most stable of the investigated M2O5-strucutres and classified it as dynamically stable. Based on this structure, different doped variants were calculated, that depict the experimentally observed phases Mo2O3.38N1.08□0.54 and Mo2O3.70N0.86□0.44 (Typ 1 or Typ 2). The calculations only lead to energetic differences of a few kJ/mol between different vacancy and anionic orderings so that a statistic ordering has to be assumed.A doping of NbON with scandium leads to a progressing stabilisation of the anatase and the rutile modification. The anatase modification is energetically preferred over the baddeleyite modification when doped with 20% scandium (equals ScNb4O7N3). The experimentally observed rutile structure is calculated to be 0.15 eV less stable. When considering the synthesis temperature of up to 1000 K as well as the configurational entropy the energy difference can change significantly, but these calculations are very time consuming.