Picosecond acoustics on iron and nickel in an internal-resistive- heated diamond anvil cell to 50 GPa and 2000 K

Abstract We have determined the longitudinal wave velocity (VP) of iron (Fe) and nickel (Ni) up to 50 GPa and 2040 K by combining picosecond acoustics and internal-resistive-heated diamond anvil cell (DAC) techniques. The high pressure–temperature VP of Fe obtained in this study are in good agreement with those previously determined by synchrotron inelastic X-ray scattering experiments. We modeled the density, temperature, and composition dependence of the VP in the Fe-Ni alloy system based on the existing data set. We found that solid Fe alloys cannot match the geophysically inferred density-VP pairs of the lunar inner core, suggesting either its absence or its anelastic relaxation. Picosecond acoustics combined with an internal-resistive-heated DAC, a technique developed for the first time in this study, will play an important role in measuring the elastic wave velocity of Fe-based alloys under terrestrial core conditions and thus constraining their chemical compositions.