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Picosecond acoustics is an optical pump-probe technique allowing to access thermoelastic properties and sound velocities of a large variety of materials under extreme conditions. Coupled with diamond anvil cells and laser heating, picosecond acoustics measurements offer the possibility to probe materials over a pressure and temperature range directly pertinent for the deep planetary interiors. In this paper we highlight the capabilities and versatility of this technique by presenting some recent applications on materials of geophysical interest. All the independent components of the elastic tensor of MgO are simultaneously determined by measurements on a single crystal at ambient conditions. Compressional sound velocity is measured at high pressure on an iron-carbon alloy and on polycrystalline argon. First laser heating test measurements performed on molybdenum at high pressure are also presented. These examples demonstrate that picosecond acoustics is a valuable alternative to already existing techniques for determining the physical properties of samples under extreme pressure and temperature conditions. [ABSTRACT FROM AUTHOR]

We performed synchrotron X-ray diffraction (XRD) and picosecond acoustic measurements on iron hydrides (FeHX) synthesized in laser-heated diamond anvil cells at pressures up to 100 GPa and 300 K. The obtained XRD data reveals that hcp FeH1.1, hcp FeH0.3, and fcc FeH1.0 were synthesized, and we determined the compressional wave velocities (VP) of the latter two phases. We discovered that the VP–density (ρ) relationships for fcc FeH1.0 changed between 8.7 and 9.2 g/cm3 (at about 60 GPa), which may be due to a magnetic transition. A comparison of the preliminary reference Earth model (PREM) with a Birch’s law extrapolation considering the temperature effect of our experimental VP–ρ results support the hypothesis of hydrogen presence in the Earth’s inner core. [ABSTRACT FROM AUTHOR]