title: Laser heating setup with high-magnification imaging for studies of physical and chemical phenomena up to ultra-high pressures in diamond anvil cells
Creators: name: Fedotenko, Timofey
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: High pressure
subject: High temperature
subject: Diamond Anvil Cells
subject: Laser heating
subject: X-ray diffraction
subject: Equation of state
subject: Transition metal carbides
subject: 530
subject: 550

description: The in situ study of matter under high-pressure and high-temperature conditions is of great interest for modern solid-state physics, chemistry, materials, geological, and planetary sciences. The diamond anvil cell (DAC) is the research instrument, enabling the broadest pressure range compared to other static pressure generation methods. Coupled with laser heating (LH), the DAC technique dramatically expands the accessible thermodynamic space. It becomes an extremely powerful tool for the synthesis and studying of novel materials at extreme conditions. Conventional LH setups, used in DAC experiments up to about 200 GPa, enable focusing of laser beams to spots on the scale of 20 µm. Experiments beyond 200 GPa require tighter focusing, at a such high pressures, the size of samples has to be reduced to a few µm. Simultaneously, to recognize the target upon heating, high-quality visualization of the sample becomes a must. We have developed a double-sided laser-heating setup with high magnification and high optical resolution imaging for in-house and synchrotron DACs experiments to address these demands. The capabilities of the setup for heating at ultra-high pressures have been tested successfully in a series of in-house experiments at multimegabar pressures. The compatibility of the setup with synchrotron techniques was demonstrated on the beamline ID15B at the European Synchrotron Radiation Facility (ESRF). We applied it for simultaneous heating and visualization of the sample using X-ray transmission microscopy (XRTM). To the best of our knowledge, this thesis work is the first demonstration of combining the LHDAC technique with XRTM. The applicability of XRTM in LHDAC for in situ melting detection was showcased in an experiment on melting of platinum Pt at 22(1) GPa. The melting point determined using this method agrees well with the literature data. Further, the developed LH setup was successfully employed for DAC experiments on carbides at moderate and ultra-high pressures, relevant for geological and materials sciences. A novel nickel carbide (Ni3C) with the cementite-type structure was synthesized in a DAC at 184(5) GPa and 3500(200) K. Its crystal structure was solved and refined using synchrotron single-crystal X-ray diffraction (SC-XRD) data. Ni3C was found to be stable down to 84(5) GPa. The calculated bulk sound velocities for Ni3C appeared to be similar to those of Fe3C and Fe7C3 up to 400 GPa. Likely, Ni's alloying doesn’t affect the elastic properties of in the Fe-Ni-C system but potentially could change the carbon distribution. Due to the stability of Ni3C at pressures above 150 GPa, it should be considered a possible candidate to carbon-bearing phases in the Earth's core along with Fe7C3. Previously not reported carbon-rich palladium carbides (PdCx, 0.015(1)