Structural investigation of iron, periclase and carbon at megabar pressures from static compression experiments using double-stage diamond anvil cell

• Applicant: Zuzana Konopkova, Ph.D.
• Subject Area: Experimental Condensed Matter Physics ; Mineralogy, Petrology and Geochemistry ; Geophysics
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 323116223

• Final Report Year: 2022

Final Report Abstract

The stress and strain evolutions of Au and Pt at pressures up to 100 GPa were investigated at strain rates up to 100 s^-1. The main analytical techniques are X-ray diffraction in standard geometry with unprecedented density of data points and line-width and line-shift analyses after Singh, 1993. Owing to the dense data coverage of the compression ramps, a close correlation between the differential stress derived from the line-width and the line-shift analyses can be found under assumption that 𝑡 ≈ 𝑝𝑚𝑎𝑥 in the line-width method as suggested for Nb. The line-shift analysis seems to be consistent with the line-width with the weight parameters α varying between 0.5-0.75, which is reasonable for both Pt and Au. A change in α from 0.5 to 0.75 at higher strain rates may suggest a change in the deformation mechanisms. Unlike in previous studies, a clear yield strength is observed in both Pt and Au, which amounts to about 2.5 GPa and 0.5 GPa, respectively. Effect of strain rates were investigated on hcp-materials as they are known to be sensitive to non-hydrostaticity, which is often manifested by their lattice parameter ratios c/a inconsistencies. dDAC compression ramps with Zn with or without PTM at moderated strain rates agree well with the literature data that shows different behavior in c/a depending on hydrostatic conditions. However, a jump compression run at 10^2 s ^-1 without any PTM agrees with a hydrostatic run suggesting that at these strain rates faster deformation mechanisms such as twinning are activated rather than dislocation slip. Spin cross-over transitions in ferropericlase Mg80Fe20O solid solution were also investigated at high temperatures in laser heating mDAC experiments. The ferropericlase sample was continuously compressed at several isotherms up to 1900 K while high density diffraction images were collected up to over 100 GPa to track deviations in unit cell volumes from an equation of state. Such deviations are cause of the softening of the bulk modulus across the spin cross-over from the high to low spin state. The local pressure of the sample is derived from the thermal equation of state of Pt that was used as a pressure calibrant. It markedly deviates from the pressure calculated from the KCl pressure medium at room temperature after accounting for the thermal pressure contribution at pressures of the supposed onset of the spin cross-over. One interpretation is that upon the spin cross-over the thermal pressure of Pt decreases due to the bulk modulus softening of the surrounding sample. Finite element modeling of the temperature and thermal pressure distribution in the laser heated DAC are currently under investigation to further understand the discrepancies between the laser heated DAC experiments and resistively heated DAC experiments and/or theoretical calculations.

Publications

• (2021). Machine Learning Applied for Spectra Classification, Computational Science and Its Applications – ICCSA 2021
  Sun et al.
  
• (2021). The High Energy Density Scientific Instrument at the European XFEL. J. Synchrotron Rad. 28, 1393-1416
  Zastrau, U et al.
  
• Investigating the effect of the compression rate on the kinetic response of diamond anvil cell experiments, Dissertation,Mathematisch-Naturwissenschaftliche Fakultät der Universität Rostock
  Christian Plückthun