Sound velocity measurements by inelastic x-ray scattering at multi-megabar pressure and high temperature: Toward constraints on the Earth’s core

The central region of the Earth is one of the frontiers of Earth science. The Earth’s inner core,
which is the deepest part of the Earth, is one of the long-standing research targets. The inner
core, which is about 3/4 the size of the Moon, is thought to be a metallic sphere composed mainly
of iron. However, many laboratory experiments and theoretical studies have shown that the
density and sound velocity of the seismic model inner core, based on the Preliminary Reference
Earth Model (PREM) [1], cannot be explained by iron alone [e.g. 2,3]. Therefore, the inner core is
thought to contain lighter elements than iron. To constrain the light elements in the inner core, it
is important to have accurate knowledge of the compression properties and sound velocity of iron
and iron-light element alloys under high pressure and high temperature conditions. Although
many high-pressure compression experiments have been conducted so far using diamond anvil
cell (DAC) [e.g. 2], the sound velocity under high pressure and high temperature has not been
well studied due to technical difficulties.
There are several methods for measuring the sound velocity of materials under static high
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pressure, such as ultrasonic measurements including picosecond and gigahertz, Brillouin
scattering, nuclear inelastic x-ray scattering (NIS), and inelastic x-ray scattering (IXS) methods.
However, NIS provides indirect measurements of compressional and shear wave velocities by
measuring the Debye velocity. The ultrasonic measurement and Brillouin scattering method are
difficult to use as a method to accurately determine the sound velocity of metals at multi-megabar
pressures due to the problems of sample size and thickness at multi-megabar pressures and/or
sample transparency. On the other hand, the IXS method does not have such problems, making
it a useful method for measuring the sound velocity of metals under high pressure.
We have been working to improve the method for measuring the sound velocity of metals at high
pressure and temperature using the IXS method. With these improvements, we have successfully
performed the sound velocity measurements of pure hcp-iron and metals up to 320 at ambient
temperature [3,4], hcp-iron-nickel alloy to 205 GPa and 2500 K, which are the most extreme
conditions ever achieved by the static sound velocity measurements, and iron-light element
compounds and other alloys at multi-megabar pressures and temperatures up to 3000 K. To
obtain weak inelastic scattering from very small and thin samples at multi-megabar pressures, we
have developed several techniques such as a newly designed Soller screen system at the
BL43LXU beamline of SPring-8 to reduce noise as much as possible [5], improved the shape of
the diamond anvil to maintain a stable pressure for long-term IXS measurements [3], and
developed the portable laser heating system (COMPAT) [6]. Here, we report on the recent
developments of our IXS measurements of sound velocity at high pressure and high temperature
and the results of our measurements.
References
[1] A. M. Dziewonski and D. L. Anderson, Phys. Earth Planet. Inter. 25 (1981) 297-356.
[2] D. Ikuta et al., Commun. Earth Environ. 2 (2021) 225.
[3] D. Ikuta et al., Nat. Commun. 13 (2022) 7211.
[4] D. Ikuta et al., Sci. Adv. 9 (2023) eadh8706.
[5] A. Q. R. Baron et al., AIP Conf. Proc. 2054 (2019) 020002.
[6] H. Fukui et al., Rev. Sci. Instrum. 84 (2013) 113902