Transport properties of Fe-Ni-Si alloys at Earth's core conditions: Insight into the viability of thermal and compositional convection
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Zhang, Youjun (ORCID:0000000153404170)↗
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Melting and phase relations of Fe-Ni-Si determined by a multi-technique approach
Many studies have suggested silicon as a candidate light element for the cores of Earth and Mercury. However, the effect of silicon on the melting temperatures of core materials and thermal profiles of cores is poorly understood, due to disagreements among melt detection techniques, uncertainties in sample pressure evolution during heating, and sparsity of studies investigating the combined effects of nickel and silicon on the phase diagram of iron. Here, in this study we develop a multi-technique approach for measuring the high-pressure melting and solid phase relations of iron alloys and apply it to Fe 0.8 Ni 0.1 Si 0.1 (Fe-11wt%Ni-5.3wt%Si), a composition compatible with recent estimates for the cores of Earth and Mercury. This approach combines results (20-83 GPa) from two atomic-level techniques: synchrotron Mossbauer spectroscopy (SMS) and synchrotron x-ray diffraction (XRD). Melting is independently detected by the loss of the Mossbauer signal, produced exclusively by solid-bound iron nuclei, and the onset of a liquid diffuse x-ray scattering signal. The use of a burst heating and background updating method for quantifying changes in the reference background during heating facilitates the determination of liquid diffuse signal onsets and leads to strong reproducibility and excellent agreement in melting temperatures determined separately by the two techniques. XRD measurements additionally constrain the hcp-fcc phase boundary and in-situ pressure evolution of the samples during heating. We apply our updated thermal pressure model to published SMS melting data on fcc-Fe and fcc-Fe 0.9 Ni 0.1 to precisely evaluate the effect of silicon on melting temperatures. We find that the addition of 10 mol% Si to Fe 0.9 Ni 0.1 reduces melting temperatures by ~250 K at low pressures (<60 GPa) and flattens the hcp-fcc phase boundary. Extrapolating our results, we constrain the location of the hcp-fcc-liquid quasi-triple point at 147±14 GPa and 3140±90 K, which implies a melting temperature reduction of 500 K compared with Fe 0.9 Ni 0.1 . The results demonstrate the advantages of combining complementary experimental techniques in investigations of melting under extreme conditions.
36 MATERIALS SCIENCE↗
The chemical composition of the cores of the terrestrial planets and the moon
Using models of the quasi-chemical theory of solutions, the activity coefficients of silicon are calculated in the melts Fe-Si, Ni-Si, and Fe-Ni-Si. The calculated free energies of solution of liquid nickel and silicon in liquid iron in the interval 0 to 1400 kbar and 1500 to 4000 K, shows that Fe-Ni-Si alloy is stable under the conditions of the outer core of the earth and the cores of the terrestrial planets. The oxidation-reduction conditions are studied, and the fugacity of oxygen in the mantles of the planets and at the core-mantle boundary are calculated. The mechanism of reduction of silicon is analyzed over a broad interval of p and T. The interaction between the matter of the core and mantle is studied, resulting in the extraction of silicon from the mantle and its solution in the material of the core. It is concluded that silicon can enter into the composition of the outer core of the earth and Venus, but probably does not enter into the composition of the cores of Mercury, Mars, and the moon, if in fact the latter possesses one.
Kuskov, O. L.↗
Materials Data on Fe3SiNi by Materials Project
Fe3NiSi is beta-derived structured and crystallizes in the cubic P2_13 space group. The structure is three-dimensional. there are six inequivalent Fe sites. In the first Fe site, Fe is bonded in a 12-coordinate geometry to six Fe, three equivalent Ni, and three equivalent Si atoms. There are two shorter (2.54 Å) and four longer (2.58 Å) Fe–Fe bond lengths. There are a spread of Fe–Ni bond distances ranging from 2.51–2.58 Å. There are a spread of Fe–Si bond distances ranging from 2.47–2.62 Å. In the second Fe site, Fe is bonded in a 12-coordinate geometry to six Fe, three equivalent Ni, and three equivalent Si atoms. There are two shorter (2.54 Å) and four longer (2.58 Å) Fe–Fe bond lengths. There are a spread of Fe–Ni bond distances ranging from 2.51–2.58 Å. There are a spread of Fe–Si bond distances ranging from 2.47–2.62 Å. In the third Fe site, Fe is bonded in a 12-coordinate geometry to six Fe, three equivalent Ni, and three equivalent Si atoms. There are two shorter (2.54 Å) and four longer (2.58 Å) Fe–Fe bond lengths. There are a spread of Fe–Ni bond distances ranging from 2.51–2.58 Å. There are a spread of Fe–Si bond distances ranging from 2.47–2.62 Å. In the fourth Fe site, Fe is bonded in a 12-coordinate geometry to six Fe, three equivalent Ni, and three equivalent Si atoms. There are one shorter (2.54 Å) and two longer (2.58 Å) Fe–Fe bond lengths. There are a spread of Fe–Ni bond distances ranging from 2.51–2.58 Å. There are a spread of Fe–Si bond distances ranging from 2.47–2.62 Å. In the fifth Fe site, Fe is bonded in a 12-coordinate geometry to six Fe, three equivalent Ni, and three equivalent Si atoms. Both Fe–Fe bond lengths are 2.58 Å. There are a spread of Fe–Ni bond distances ranging from 2.51–2.58 Å. There are a spread of Fe–Si bond distances ranging from 2.47–2.62 Å. In the sixth Fe site, Fe is bonded in a 12-coordinate geometry to six Fe, three equivalent Ni, and three equivalent Si atoms. There are a spread of Fe–Ni bond distances ranging from 2.51–2.58 Å. There are a spread of Fe–Si bond distances ranging from 2.47–2.62 Å. Ni is bonded to nine Fe and three equivalent Si atoms to form NiFe9Si3 cuboctahedra that share corners with six equivalent NiFe9Si3 cuboctahedra, corners with nine equivalent SiFe9Ni3 cuboctahedra, faces with four equivalent SiFe9Ni3 cuboctahedra, and faces with six equivalent NiFe9Si3 cuboctahedra. All Ni–Si bond lengths are 2.28 Å. Si is bonded to nine Fe and three equivalent Ni atoms to form SiFe9Ni3 cuboctahedra that share corners with six equivalent SiFe9Ni3 cuboctahedra, corners with nine equivalent NiFe9Si3 cuboctahedra, faces with four equivalent NiFe9Si3 cuboctahedra, and faces with six equivalent SiFe9Ni3 cuboctahedra.
36 MATERIALS SCIENCE↗
Materials Data on Fe3Si2Ni3 by Materials Project
Fe3Ni3Si2 is alpha La-derived structured and crystallizes in the orthorhombic Pmmm space group. The structure is three-dimensional. there are two inequivalent Fe sites. In the first Fe site, Fe is bonded to two equivalent Fe, six Ni, and four equivalent Si atoms to form FeFe2Si4Ni6 cuboctahedra that share corners with twelve equivalent FeFe2Si4Ni6 cuboctahedra, edges with four equivalent FeFe4Si4Ni4 cuboctahedra, edges with eight equivalent SiFe6Ni6 cuboctahedra, edges with twelve NiFe4Si4Ni4 cuboctahedra, faces with four equivalent SiFe6Ni6 cuboctahedra, faces with six NiFe4Si4Ni4 cuboctahedra, and faces with eight FeFe2Si4Ni6 cuboctahedra. Both Fe–Fe bond lengths are 2.45 Å. There are a spread of Fe–Ni bond distances ranging from 2.49–2.54 Å. All Fe–Si bond lengths are 2.51 Å. In the second Fe site, Fe is bonded to four equivalent Fe, four equivalent Ni, and four equivalent Si atoms to form FeFe4Si4Ni4 cuboctahedra that share corners with four equivalent FeFe4Si4Ni4 cuboctahedra, corners with eight equivalent NiFe4Si4Ni4 cuboctahedra, edges with eight equivalent FeFe2Si4Ni6 cuboctahedra, edges with eight equivalent NiFe4Si4Ni4 cuboctahedra, edges with eight equivalent SiFe6Ni6 cuboctahedra, faces with four equivalent SiFe6Ni6 cuboctahedra, faces with six NiFe4Si4Ni4 cuboctahedra, and faces with eight FeFe2Si4Ni6 cuboctahedra. All Fe–Ni bond lengths are 2.51 Å. All Fe–Si bond lengths are 2.54 Å. There are three inequivalent Ni sites. In the first Ni site, Ni is bonded to four equivalent Fe, four equivalent Ni, and four equivalent Si atoms to form NiFe4Si4Ni4 cuboctahedra that share corners with four equivalent NiFe4Si4Ni4 cuboctahedra, corners with eight equivalent FeFe4Si4Ni4 cuboctahedra, edges with eight equivalent FeFe2Si4Ni6 cuboctahedra, edges with eight equivalent NiFe8Si4 cuboctahedra, edges with eight equivalent SiFe6Ni6 cuboctahedra, faces with four equivalent SiFe6Ni6 cuboctahedra, faces with six FeFe2Si4Ni6 cuboctahedra, and faces with eight NiFe4Si4Ni4 cuboctahedra. All Ni–Ni bond lengths are 2.51 Å. All Ni–Si bond lengths are 2.51 Å. In the second Ni site, Ni is bonded to eight Fe and four equivalent Si atoms to form NiFe8Si4 cuboctahedra that share corners with twelve NiFe8Si4 cuboctahedra, edges with eight equivalent FeFe2Si4Ni6 cuboctahedra, edges with eight equivalent NiFe4Si4Ni4 cuboctahedra, edges with eight equivalent SiFe6Ni6 cuboctahedra, faces with four equivalent SiFe6Ni6 cuboctahedra, faces with six NiFe8Si4 cuboctahedra, and faces with eight FeFe2Si4Ni6 cuboctahedra. All Ni–Si bond lengths are 2.48 Å. In the third Ni site, Ni is bonded to four equivalent Fe, four equivalent Ni, and four equivalent Si atoms to form NiFe4Si4Ni4 cuboctahedra that share corners with twelve NiFe8Si4 cuboctahedra, edges with eight equivalent SiFe6Ni6 cuboctahedra, edges with sixteen FeFe2Si4Ni6 cuboctahedra, faces with four equivalent FeFe2Si4Ni6 cuboctahedra, faces with four equivalent SiFe6Ni6 cuboctahedra, and faces with ten NiFe4Si4Ni4 cuboctahedra. All Ni–Si bond lengths are 2.46 Å. Si is bonded to six Fe and six Ni atoms to form SiFe6Ni6 cuboctahedra that share corners with twelve equivalent SiFe6Ni6 cuboctahedra, edges with twelve FeFe2Si4Ni6 cuboctahedra, edges with twelve NiFe4Si4Ni4 cuboctahedra, faces with six FeFe2Si4Ni6 cuboctahedra, faces with six NiFe4Si4Ni4 cuboctahedra, and faces with six equivalent SiFe6Ni6 cuboctahedra.
36 MATERIALS SCIENCE↗
Materials Data on Fe2SiNi by Materials Project
Fe2NiSi is Tungsten-derived structured and crystallizes in the cubic F-43m space group. The structure is three-dimensional. there are two inequivalent Fe sites. In the first Fe site, Fe is bonded in a 8-coordinate geometry to four equivalent Fe, four equivalent Ni, and six equivalent Si atoms. All Fe–Fe bond lengths are 2.44 Å. All Fe–Ni bond lengths are 2.44 Å. All Fe–Si bond lengths are 2.82 Å. In the second Fe site, Fe is bonded in a 8-coordinate geometry to four equivalent Fe, six equivalent Ni, and four equivalent Si atoms. All Fe–Ni bond lengths are 2.82 Å. All Fe–Si bond lengths are 2.44 Å. Ni is bonded in a distorted body-centered cubic geometry to ten Fe and four equivalent Si atoms. All Ni–Si bond lengths are 2.44 Å. Si is bonded in a distorted body-centered cubic geometry to ten Fe and four equivalent Ni atoms.
36 MATERIALS SCIENCE↗