3:00 PM - 3:15 PM
[SIT16-11] Determination of temperature dependence of elastic wave velocity of high-pressure fcc iron using Picosecond Acoustics
Keywords:physical property at high pressure and temperature conditions, elastic wave velocity measurement, Internally-resistance heated diamond anvil cell
The core of terrestrial planets and satellites are thought to be iron-based alloys, but their detailed composition is still unknown, even on Earth. The standard method for constraining the composition of deep-planetary-interior is to compare experimental data of elastic wave velocity at high pressure(P) and temperature(T) conditions with a one-dimensional seismic wave velocity profile. Seismic wave velocities vary with composition and crystal structure. Even in the same phase, it can be affected by P-T variation. Therefore, it is desirable to measure the elastic wave velocities under high P-T conditions that are equivalent to those of the planetary core.
Static compressional experiments using a diamond anvil cell (DAC) showed that pure iron has a hexagonal close-packed (hcp) structure under the Earth's inner core conditions (Tateno et al. (2010)). Therefore, elastic wave velocity measurements of hcp-iron at high P-T conditions have been intensively performed with Inelastic X-ray Scattering (IXS) techniques or Nuclear Resonant Inelastic X-ray Scattering (NRIXS). So far, the maximum experimental P-T conditions of the elastic wave velocity in hcp iron are 327 GPa (Ikuta et al. (2022)) and 3000 K (Sakamaki et al. (2016)), respectively. On the other hand, terrestrial planets such as Mercury, Mars, and the Moon are thought to have lower P or T in their core than the Earth does. Mercury’s core is 8~40 GPa, 1700~2200 K (Chen et al. (2008), Margot et al. (2017)), Mars’ is 24~42 GPa, 2000~2600 K (Bertka and Fei (1998), Fei and Bertka (2005)), and the Moon’s is 5~6 GPa, 1300~1900 K (Wieczorek et al. (2006)). Under these P-T conditions, the face-centered cubic (fcc) structure is stable for iron (Tsujino et al., 2013).
In contrast to hcp-iron, there are few reported cases of elastic wave velocity measurements for fcc-iron: Debye sound velocity measurements at 6 GPa and up to 920 K (Shen et al. (2008)), Inelastic Neutron Scattering (NIS) measurements at ambient P and 1428 K (Zarestky (1987)), IXS measurements up to 19 GPa and 1100 K (Antonangeli et al. (2015)). None of these are comparable to the P-T values of the planets’ core mentioned above. One reason for this paucity of reports is that light scattering experiments on iron, which is an opaque material, require the use of monochromatic x-ray in large synchrotron radiation facilities, making it difficult to obtain data easily.
In this study, therefore, we developed the laboratory-based optical system for the Picosecond Acoustics (PA)-method (Decremps et al. (2014), Wakamatsu et al. (2022)), which has an advantage of short time measurement, regardless of whether the sample is transparent or opaque. Sample was loaded into an Internally-Resistance Heated DAC, which allows spatially and temporally stable high-temperature heating (Zha et al. (2003), Sinmyo et al. (2019)). A femtosecond-order pulsed laser beam was radiated from one side of the sample along the compressional axis of the DAC to generate compressional waves in a thin sample, and the travel time of the propagating wave was measured. The temperature dependence of elastic wave velocity was estimated from the travel time variation with the temperature of the sample. The high-temperature experiment of the PA-method is limited to the measurement of a Molybdenum foil combined with a laser-heated DAC (Boccato et al. (2022)). Therefore, this study is the first example of determining the temperature dependence of the elastic wave velocity for most abundant material in the planetary core using the PA-method.
Static compressional experiments using a diamond anvil cell (DAC) showed that pure iron has a hexagonal close-packed (hcp) structure under the Earth's inner core conditions (Tateno et al. (2010)). Therefore, elastic wave velocity measurements of hcp-iron at high P-T conditions have been intensively performed with Inelastic X-ray Scattering (IXS) techniques or Nuclear Resonant Inelastic X-ray Scattering (NRIXS). So far, the maximum experimental P-T conditions of the elastic wave velocity in hcp iron are 327 GPa (Ikuta et al. (2022)) and 3000 K (Sakamaki et al. (2016)), respectively. On the other hand, terrestrial planets such as Mercury, Mars, and the Moon are thought to have lower P or T in their core than the Earth does. Mercury’s core is 8~40 GPa, 1700~2200 K (Chen et al. (2008), Margot et al. (2017)), Mars’ is 24~42 GPa, 2000~2600 K (Bertka and Fei (1998), Fei and Bertka (2005)), and the Moon’s is 5~6 GPa, 1300~1900 K (Wieczorek et al. (2006)). Under these P-T conditions, the face-centered cubic (fcc) structure is stable for iron (Tsujino et al., 2013).
In contrast to hcp-iron, there are few reported cases of elastic wave velocity measurements for fcc-iron: Debye sound velocity measurements at 6 GPa and up to 920 K (Shen et al. (2008)), Inelastic Neutron Scattering (NIS) measurements at ambient P and 1428 K (Zarestky (1987)), IXS measurements up to 19 GPa and 1100 K (Antonangeli et al. (2015)). None of these are comparable to the P-T values of the planets’ core mentioned above. One reason for this paucity of reports is that light scattering experiments on iron, which is an opaque material, require the use of monochromatic x-ray in large synchrotron radiation facilities, making it difficult to obtain data easily.
In this study, therefore, we developed the laboratory-based optical system for the Picosecond Acoustics (PA)-method (Decremps et al. (2014), Wakamatsu et al. (2022)), which has an advantage of short time measurement, regardless of whether the sample is transparent or opaque. Sample was loaded into an Internally-Resistance Heated DAC, which allows spatially and temporally stable high-temperature heating (Zha et al. (2003), Sinmyo et al. (2019)). A femtosecond-order pulsed laser beam was radiated from one side of the sample along the compressional axis of the DAC to generate compressional waves in a thin sample, and the travel time of the propagating wave was measured. The temperature dependence of elastic wave velocity was estimated from the travel time variation with the temperature of the sample. The high-temperature experiment of the PA-method is limited to the measurement of a Molybdenum foil combined with a laser-heated DAC (Boccato et al. (2022)). Therefore, this study is the first example of determining the temperature dependence of the elastic wave velocity for most abundant material in the planetary core using the PA-method.