Experimental studies of metal-nitrides in extreme conditions
Creators: name: University Of California Office Of The President
Publisher: name: University of California Office of the President
publisherIdentifier: https://ror.org/00dmfq477
publisherIdentifierScheme: ROR
Year: 2024

description: Since plate tectonic theory was formulated, it has become widely accepted that nearly all geological activities at Earth¡¯s surface, including mountain building, earthquakes, volcanic eruptions, and even the composition of the atmosphere, are generated by processes occurring at depth. Although most of the Earth is inaccessible for sampling by drilling or any other direct means, we have ideas about what minerals might exist at depth from studies of tectonic fragments of mantle slices and xenoliths, meteorites, stable isotope signatures, and ultra-high-pressure phase-equilibrium experiments. We have recently discovered minerals from the nitride group (TiN-osbornite and cubic boron nitride) as inclusions in high-pressure polymorphs of Si2O associated with FeTi alloy. These minerals were found within the mantle section of the Tebetan ophiolite, a former mid-oceanic upwelling zone that now marks the geological boundary between India and the Asian continent where they collided 65 Myr. The sample was brought to the surface from a depth of &gt;300 km, and its ¦Ä15N values suggest that the nitrogen originated from a primordial reservoir. TiN and other nitrides often occur in enstatite meteorites, a parental material for the Earth¡¯s formation, and in iron meteorites, which contain material that may be represented in the Earth¡¯s core. Because of the very negative values of ¦Ä15N (-10¡ë) from the Tibetan TiN, we hypothesize that primordial N could have leaked from the Earth¡¯s core. Iron has been known for a decade to be too dense compared with seismically estimated core density, and this has instigated speculations that the core must be composed of light elements. With our discovery of terrestrial nitrides we extend an existing hypothesis that nitrogen can be a candidate for the light alloying element concentrated in the volumetrically minor inner-core material. If the hypothesis is correct, global models of the Earth¡¯s composition will need to be reworked, at a minimum. In attempting to understand these rocks further, the following major questions must be addressed: (1) What is the significance of this finding of nitrides in mantle rocks in a zone of upwelling? (2) Where does the nitrogen come from? (3) Are metal nitrides stable in the Earth¡¯s mantle-core regions? We propose experimental studies of nitrides at extreme pressures (5¨C130 GPa) and temperatures (1000¨C3000 oC) in a multi-anvil apparatus and a diamond-anvil cell device coupled with synchrotron micro-X-ray in situ studies of run products. Establishing the phase boundaries for TiN and other metal nitrides at high P and T and their equations of state will begin to outline whether nitrogen can be a significant constituent in the Earth¡¯s mantle and core. Further study of N behavior, the ¦Ä15N value in terrestrial nitrides, and potential N isotope fractionation events at high/extreme pressures could be of major importance for our knowledge of the deep Earth. Other metal nitrides, including Li-nitride, will also be synthesized at high pressures and temperatures to explore the synthesis of high-density metal nitrides by reaction of the nitrogen pressure medium with elemental metal substrates. This part of the project has a high technological significance because high-density nitrides show promise for both ceramic and electronic applications. Lithium nitride (LiN3) is a potential material for use in fuel cells and hydrogen storage, which fits with the mission of the Lawrence Livermore National Laboratory (LLNL) and the strategic interests of society. The proposed research will be carried out in collaboration with colleagues from LLNL and will support two graduate students. The UCR team will work at LLNL with the most powerful transmission electron microscope of the TitanTM 80-300 series. It brings research to the highest level of atomic detail available today¡ªdown to the sub 0‡3ngstrom level¡ªproviding critical data for process development and materials research. The electron energy-loss spectrometer (EELS) available at LLNL, in conjunction with the TitanTM, will be a superb instrument for detection of nitrogen content in the experimental samples. The synthesis of nitrides will be performed at UC Riverside and in the Advanced Photon Source facilities at Argonne National Laboratory; their equipment represents state-of-the-art scientific instrumentation and technologies. The proposed theme is not restricted by classification or deemed export rules.