Synthesis and Compression study of orthorhombic $Fe_7(C,Si)_3$: A possible constituent of the Earths core

Abstract The orthorhombic phase of Si-doped Fe carbide is synthesized at high pressures and temperatures using laser-heated diamond anvil cell (LHDAC), followed by its characterization using X-ray diffraction (XRD) measurements, Transmission Electron Microscopy (TEM), and Raman spectroscopy. High-pressure XRD measurements are carried out up to about 104 GPa at room temperature for determination of the equation of state (EOS) parameters of the synthesized sample. No evidence of structural transition is observed, though two anomalies are found in the compression behaviour of our sample at about 28 and 78 GPa, respectively. Pressure evolution of isothermal bulk modulus shows elastic stiffening around 28 GPa followed by softening around 78 GPa. These anomalies are possibly related to two different magnetic transitions driven by pressure-induced anisotropic strain in the unit cell. Extrapolation of the density profile of our study to the inner core conditions agrees very well with PREM data with an uncertainty of about 3-4%. We have estimate bulk modulus value seems to be 8-9% less than that of PREM data in the shown pressure range and is best matched in comparison to other reported values for the non- magnetic phase.
Keywords: Laser heated diamond anvil cell, orthorhombic Si-doped Fe7C3, elastic anomalies, Earth’s inner core density, crystal structure

1 Introduction For several decades, the composition of Earth’s core has been under extensive debate in scientific communities. Amongst several elements, Iron (Fe) and its alloys with Nickel (Ni) have been predicted to be the main components of the Earth’s core from X-ray diffraction experiments at extreme pressures. But densities (Mao et al., 1990; Dubrovinsky et al., 2000) of these materials are found to be significantly higher than that of Earth’s core estimated from seismic observations (Dziewonski & Anderson 1981; Stevenson 1981). Given the above results, it has been suggested that a few percentages of light elements may be present along with Fe or Fe − Ni alloy (Birch 1952; Wood 1993; Poirier 1994; Li et al., 2002; Li & Fei 2003). Initially, Carbon (C) was considered to be the leading light element along with Fe in the form Fe3C at Earth’s core due to its high solar abundance, chemical affinity to iron at low pressures, and overall ability to lower the density of pure Fe or Fe − Ni alloy (Wood 1993). Sata et al. (2010) show that C is the most dominant constituent along with Fe among other light elements such as Si, O, S due to its minimal density deficit concerning the PREM data at inner core conditions (Dziewonski & Anderson 1981). However, theoretical studies predicted a non-magnetic Fe3C with a larger bulk modulus compared to the PREM data (Vočadlo et al., 2002). High pressure and high-temperature experiments using both multi-anvil (MA) cell and LHDAC on Fe − C system resulted in the formation of a new iron-carbide phase with chemical formula Fe7C3 at about 1500°C and 10 GPa, which is predicted to be a potential candidate for the solid inner core (Nakajima et al., 2009; Nakajima et al., 2011; Mookherjee et al., 2011; Chen et al., 2012; Chen et al., 2014; Prescher et al., 2015; Liu et al., 2016). Several research groups indexed the synthesized phase to the hexagonal structure (Nakajima et al., 2011; Chen et al., 2012). Electronic structure calculations by Mookherjee et al. (2011) on hexagonal Fe7C3 at Earth’s core conditions proposed around 7% less density as compared to PREM data. Experimental work by Chen et al. (2012) proposed the density of hexagonal Fe7C3 be about 5- 10% lower concerning the PREM data at inner core pressure and temperature range 5000-7000 K. Recently, Prescher et al. (2015) synthesized an orthorhombic phase of Fe7C3 using MA apparatus. A theoretical simulation study by Das et al. (2017) comparing the hexagonal, orthorhombic pure phases of Fe7C3 and silicon (Si) doped Fe7C3 showed that values of density and Poisson’s ratio of orthorhombic (o)-Fe7(C, Si)3 at the inner core to be very close to the PREM data. The above study proposed o-Fe7(C, Si)3 with 3.2 wt % of Si at C cite to be one of the most important components of the Earth’s inner core. Depending on the current trends of the possible candidates for the core material, we have successfully synthesized the orthorhombic phase of Si-doped Fe7C3 at high pressures and high temperatures using the LHDAC facility present in our laboratory. We have

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