title: Tailored perovskite-type oxynitride semiconductors and oxides with advanced physical properties
Creators: name: Bubeck, Cora Maren
Publisher: name: UNSPECIFIED
Year: 2023

description: Perovskite materials (ABX3) reveal a surprisingly large variety of technologically interesting, highly advanced properties for application in solar cells, spin-optoelectronics or magnetic field sensors. In particular, perovskite-type oxynitrides AB(O,N)3 are a well-known class of materials for visible light-driven applications and as inorganic pigments. In the last few decades, their range of applications e.g. in solar water splitting (SWS) have been expanded by the discovery of a great number of before unknown materials. However, the formation of such high-applicable materials was not totally clarified and with it the targeted tuning of their physical properties. To tailor the physical properties in perovskite-type oxynitrides substitutions on the A- and B-site are common, whereas the anionic site (X-site) is less explored. In the first part of the cumulative dissertation, the formation processes of LaTaIVO2N and LaTaVON2 from the respective oxide precursors were elucidated. Additionally, the desired oxidation state of Ta and the nitrogen content in the compounds was adjusted. This opened up new perspectives for the understanding of the ammonolysis process in general, which is used for the formation of perovskite-type oxynitrides from oxide precursors. The here synthesized perovskite-type oxynitrides are promising for light-driven applications because of their measured optical bandgap and low optically active defect concentration. Additionally, the range of potentially suited candidates is expanded by degenerated semiconducting oxynitrides. In the second part, in addition to the nitrogen content and the oxidation state of Ta in La1–xYxTaIVO2N (x = 0, 0.1, 0.25, 0.3, and 1.0) the cationic ratio between Y3+ and La3+ (A-site substitution) was modified. This resulted in controlled physical properties such as an adjusted optical and effective band gap size and a significant charge carrier transport rate. These are important features for SWS and the orthorhombic strain is added to the key descriptors for the band gap size in perovskite-type oxynitrides. In the third part, instead of an A-site substitution a B-site substitution of Taz+ for Coz+ in LaTa(O,N)3 was applied. This led to the previously unknown perovskite-type oxynitrides LaTa1–xCox(O,N)3–δ (x = 0.01, 0.03, and 0.05). The material exhibited a ferromagnetic order with a Curie temperature exceeding 600 K. The synthesized material corresponds – to the best of one’s knowledge – to the first diluted ferromagnetic semiconducting perovskite-type oxynitride. Hence, by substitution of a tiny amount of magnetic B-site cations (≤ 1 at%) in the pristine diamagnetic LaTa(O,N)3 physical properties such as ferromagnetism can be tuned. In the fourth part, through a targeted B-site substitution in La0.6Ca0.4Co1–xFexO3−δ (x =0, 0.3, 0.5, 0.7, 1) physical properties such as CO2 adsorption abilities, oxygen permeability, and electrical conductivity were tuned. These are – amongst other features – important for the application in carbon capture and utilization. The variation of the Fe/Co ratio led to an improvement of the measured oxygen permeation flux. Furthermore, conducted DFT calculations opened up the possibility to determine the effect of the Fe/Co ratio on the oxygen migration behavior and formation energy of the found oxygen vacancies. The results shown in this thesis can be used to synthesize further targeted perovskite-type oxynitrides and oxides exhibiting advanced physical properties for future applications.