Mechanismen der Bildung metastabiler Hydride und Nitride mittels in situ-Neutronenbeugung von Fest-Gas-Reaktionen

点击翻译按钮获取中文摘要

Understanding of reaction pathways is a fundamental basis in the synthesis of metastable compounds. Therefore, we have developed gas-pressure and gas-flow cells for in situ neutron diffraction aiming on the reaction of solids with hydrogen (deuterium) or ammonia gas. With the former 160 bar gas pressure and 400 °C are accessible in combination with extremely low diffraction background due to proper orientation of a sapphire single crystal tube as a sample container. Higher temperatures (1000 °C) are achieved using silica tubes like in the gas-flow cell. Both are heated with a two-sided laser system and the temperature is measured with a calibrated pyrometer. In situ neutron diffraction on solid-gas reactions with hydrogen or ammonia gas with a time resolution of one minute is now well established with both cells on the high-intensity diffractometer D20 at the ILL in Grenoble, France. A variety of systems was studied, amongst them the formation of deuterides of palladium, Zintl phases, intermetallic Li-Al compounds, lanthanide intermetallics, VN, V2N, and of Fe4N via intermediate Fe3N. For palladium rich intermetallic phases a complete reaction mechanism could be proposed, which allowed the planned synthesis of new metastable compounds. The successful concept of our in situ gas cells will be optimized in view of the temperature-pressure range (seal and container materials), automated pressure-temperature programs and installing them on the diffractometer SPODI at the FRM-II, Garching. Using the known reaction mechanism for palladium rich intermetallics, new metastable hydrides will be synthesized for metals M = Tl1-xPbx, In1-xSnx, As, Sb, Se, Mn. Metastable intermediates in the formation of mixed-valent ytterbium hydrides and the hydrogenation of light-weight intermetallics of interest for hydrogen storage will be studied as well. For nitrides the knowledge already acquired on vanadium and iron will be deepened and extended to chromium, manganese, cobalt, nickel and copper, in order to yield and structurally characterize metastable compounds, such as Fe16N2, Fe8N or Co3N, some of them are highly interesting in view of their magnetic properties. Other synthetic strategies to metastable nitrides include the use of predissociated ammonia, decomposition of precursor materials and the ammonolysis of metal hydrides.The formation of metastable oxides and oxide nitrides will be studied, in order to widen the range of materials and to strengthen collaboration within the SPP. In situ neutron diffraction on the ammonolysis of Ta2O5 will give a detailed picture of the formation of the photocatalyst Ta3N5 via transiently occurring TaON and its metastable modification. We will also follow the formation of metastable bixbyite type V2O3 and the formation of black phosphorous. Experimental studies will be backed up by quantum mechanical calculations in cooperation within and outside the SPP.