The accurate measurement of experimentally generated ultrahigh pressure is difficult. In the past scientists compared values of static and dynamic pressures to calibrate such high pressure. However, this technique introduces a large range of uncertainty, as dynamic experiments also tend to significantly increase temperature. High pressures exist in all planetary interiors, so improvements of experimental techniques to accurately replicate planetary material behavior is critical to modeling other worlds. But, because of difficulties in accurately measuring pressure, information about phase and material boundaries in planetary interiors (e.g., core-mantle) remains uncertain.
A collaborative team of scientists at the Carnegie Institution of Washington and the University of Chicago at the Advanced Photon Source (Argonne National Laboratory) has made substantial inroads into this problem. By performing numerous time-consuming experiments on simultaneous measurements of the sound velocities and density of cubic silicon carbide (SiC), they have been able to make accurate measurements at pressures representative of the conditions of Earth’s lower mantle. The pressure calibrant SiC was carefully chosen, and is sufficiently sensitive to pressure variation, physically and chemically stable, and convenient for accurate Brillouin and x-ray diffraction measurements. Such measurements determine the pressure directly without relying on any assumptions about material properties.
As a result of this work, an accurate primary pressure scale has been constructed which differs substantially from those previously proposed. This work is an important step toward constructing an accurate pressure scale to higher pressures (>100 GPa) and to high temperatures (up to 4000 K), where pressure measurements currently rely on extrapolations. As Brillouin spectroscopy, which is an important part of this work, has limitations at ultrahigh pressure, the team is planning to extend its calibration to using an ultrafast (femtosecond) laser technique called acoustic interferometry, the development of which is partially supported by DCO at the Geophysical Laboratory. This work could be an important step toward acquiring accurate thermodynamic data on Earth’s carbon-bearing minerals and volatiles, which are crucial for understanding the deep Earth carbon cycle.
Photo courtesy of the author: The combined synchrotron diffraction-Brillouin-Raman optical system at GSECARS, Sector 13 of APS, that was used in this work.
Contributed by Alexander Goncharov, Carnegie Institution of Washington, Washington, DC, USA