Sound velocity of iron up to 152 GPa by picosecond acoustics in diamond anvil cell

Abstract

AbstractHigh‐pressure method combining diamond anvil cell with picosecond ultrasonics technique is demonstrated to be a very suitable tool to measure the acoustic properties of iron up to 152 GPa. Such innovative approach allows to measure directly the longitudinal sound velocity under pressure of hundreds of GPa in laboratory, overcoming most of the drawbacks of traditional techniques. The very high accuracy, comparable to piezoacoustics technique, allows to observe the kink in elastic properties at the body‐centered cubic–hexagonal close packed transition and to show with a good confidence that the Birch's law still stands up to 1.5 Mbar and ambient temperature. The linear extrapolation of the measured sound velocities versus densities of hcp iron is out of the preliminary reference Earth model, arguing for alloying effects or anharmonic high‐temperature effects. A comparison between our measurements and shock wave experiments allowed us to quantify temperature corrections at constant pressure in ~−0.35 and ~−0.30 m s−1/K at 100 and 150 GPa, respectively. More in general, the here‐presented technique allows detailed elastic and viscoelastic studies under extreme thermodynamic conditions on a wide variety of systems as liquids, crystalline, or polycrystalline solids, metallic or not, with very broad applications in Earth and planetary science.