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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on MgSi by Materials Project

MgSi is Magnesium tetraboride-like structured and crystallizes in the triclinic P-1 space group. The structure is three-dimensional. there are three inequivalent Mg sites. In the first Mg site, Mg is bonded in a 6-coordinate geometry to six Si atoms. There are a spread of Mg–Si bond distances ranging from 2.73–2.89 Å. In the second Mg site, Mg is bonded in a 6-coordinate geometry to six Si atoms. There are a spread of Mg–Si bond distances ranging from 2.76–3.01 Å. In the third Mg site, Mg is bonded in a 5-coordinate geometry to five Si atoms. There are a spread of Mg–Si bond distances ranging from 2.77–2.86 Å. There are three inequivalent Si sites. In the first Si site, Si is bonded in a 8-coordinate geometry to six Mg and two Si atoms. There are one shorter (2.37 Å) and one longer (2.42 Å) Si–Si bond lengths. In the second Si site, Si is bonded in a 7-coordinate geometry to five Mg and two Si atoms. There are one shorter (2.40 Å) and one longer (2.44 Å) Si–Si bond lengths. In the third Si site, Si is bonded in a 8-coordinate geometry to six Mg and two Si atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ce(MgSi)2 by Materials Project

CeMg2Si2 crystallizes in the tetragonal P4/mmm space group. The structure is three-dimensional. Mg2+ is bonded in a square co-planar geometry to four equivalent Si4- atoms. All Mg–Si bond lengths are 2.75 Å. Ce4+ is bonded to eight equivalent Si4- atoms to form a mixture of edge and face-sharing CeSi8 hexagonal bipyramids. All Ce–Si bond lengths are 3.19 Å. Si4- is bonded in a 9-coordinate geometry to four equivalent Mg2+, four equivalent Ce4+, and one Si4- atom. The Si–Si bond length is 2.29 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ba(MgSi)2 by Materials Project

BaMg2Si2 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Ba is bonded in a 8-coordinate geometry to eight equivalent Mg and eight equivalent Si atoms. All Ba–Mg bond lengths are 3.62 Å. All Ba–Si bond lengths are 3.53 Å. Mg is bonded to four equivalent Ba and four equivalent Si atoms to form a mixture of distorted edge, face, and corner-sharing MgBa4Si4 tetrahedra. All Mg–Si bond lengths are 2.79 Å. Si is bonded in a 9-coordinate geometry to four equivalent Ba, four equivalent Mg, and one Si atom. The Si–Si bond length is 2.48 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgSi(HO2)2 by Materials Project

MgSiO2(OH)2 crystallizes in the monoclinic P2/m space group. The structure is three-dimensional. Mg2+ is bonded to six O2- atoms to form MgO6 octahedra that share corners with eight equivalent SiO6 octahedra and edges with two equivalent MgO6 octahedra. The corner-sharing octahedra tilt angles range from 52–55°. There are four shorter (2.05 Å) and two longer (2.18 Å) Mg–O bond lengths. Si4+ is bonded to six O2- atoms to form SiO6 octahedra that share corners with eight equivalent MgO6 octahedra and edges with two equivalent SiO6 octahedra. The corner-sharing octahedra tilt angles range from 52–55°. There is two shorter (1.76 Å) and four longer (1.88 Å) Si–O bond length. There are two inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a linear geometry to two equivalent O2- atoms. Both H–O bond lengths are 1.22 Å. In the second H1+ site, H1+ is bonded in a linear geometry to two equivalent O2- atoms. Both H–O bond lengths are 1.21 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a 4-coordinate geometry to two equivalent Mg2+, one Si4+, and one H1+ atom. In the second O2- site, O2- is bonded in a 1-coordinate geometry to one Mg2+, two equivalent Si4+, and one H1+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgSi(HO2)2 by Materials Project

MgSiO2(OH)2 is Pb(Zr_(1-x)Ti_x)O3-derived structured and crystallizes in the monoclinic Pm space group. The structure is three-dimensional. Mg2+ is bonded to six O2- atoms to form distorted MgO6 octahedra that share corners with eight equivalent SiO6 octahedra and edges with two equivalent MgO6 octahedra. The corner-sharing octahedra tilt angles range from 50–57°. There are a spread of Mg–O bond distances ranging from 2.00–2.20 Å. Si4+ is bonded to six O2- atoms to form SiO6 octahedra that share corners with eight equivalent MgO6 octahedra and edges with two equivalent SiO6 octahedra. The corner-sharing octahedra tilt angles range from 50–57°. There are a spread of Si–O bond distances ranging from 1.71–1.94 Å. There are two inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a linear geometry to two O2- atoms. There is one shorter (1.05 Å) and one longer (1.50 Å) H–O bond length. In the second H1+ site, H1+ is bonded in a linear geometry to two O2- atoms. There is one shorter (1.06 Å) and one longer (1.44 Å) H–O bond length. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to two equivalent Mg2+, one Si4+, and one H1+ atom. In the second O2- site, O2- is bonded in a distorted single-bond geometry to one Mg2+, two equivalent Si4+, and one H1+ atom. In the third O2- site, O2- is bonded in a distorted single-bond geometry to two equivalent Mg2+, one Si4+, and one H1+ atom. In the fourth O2- site, O2- is bonded in a 4-coordinate geometry to one Mg2+, two equivalent Si4+, and one H1+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Anharmonic thermodynamic properties and phase boundary across the postperovskite transition in MgSi O 3

To address the effects of lattice anharmonicity across the perovskite to postperovskite transition in MgSiO 3 , we conduct calculations using the phonon quasiparticle (PHQ) approach. The PHQ is based on abinitio molecular dynamics and, in principle, captures full anharmonicity. Free energies in the thermodynamic limit (N → ∞) are computed using temperature-dependent quasiparticle dispersions within the phonon gas model. Systematic results on anharmonic thermodynamic properties and phase boundary are reported. Both the local density approximation and the generalized gradient approximation calculations are performed to provide confident constraints on these properties. Anharmonic effects are demonstrated by comparing results with those obtained using the quasiharmonic approximation (QHA). The inadequacy of the QHA is indicated by its overestimation of thermal expansivity and thermodynamic Grüneisen parameter and its converged isochoric heat capacity in the high-temperature limit. The PHQ phase boundary has a Clapeyron slope (dP/dT) that increases with temperature. This result contrasts with the nearly zero curvature of the QHA phase boundary. Anharmonicity bends the phase boundary to lower temperatures at high pressures. Implications for the double-crossing of the phase boundary by the mantle geotherm are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Li2MgSi by Materials Project

Li2MgSi crystallizes in the cubic Fm-3m space group. The structure is three-dimensional and consists of eight lithium molecules and one Li7(MgSi)4 framework. In the Li7(MgSi)4 framework, there are two inequivalent Li1+ sites. In the first Li1+ site, Li1+ is bonded to four Si4- atoms to form distorted LiSi4 tetrahedra that share corners with six equivalent MgSi4 tetrahedra, corners with ten equivalent LiSi4 tetrahedra, edges with three equivalent LiSi4 tetrahedra, and edges with three equivalent MgSi4 tetrahedra. There are three shorter (2.75 Å) and one longer (2.77 Å) Li–Si bond lengths. In the second Li1+ site, Li1+ is bonded in a distorted single-bond geometry to one Si4- atom. The Li–Si bond length is 2.78 Å. Mg2+ is bonded to four Si4- atoms to form MgSi4 tetrahedra that share corners with six equivalent LiSi4 tetrahedra, corners with ten equivalent MgSi4 tetrahedra, edges with three equivalent LiSi4 tetrahedra, and edges with three equivalent MgSi4 tetrahedra. There are three shorter (2.75 Å) and one longer (2.77 Å) Mg–Si bond lengths. There are three inequivalent Si4- sites. In the first Si4- site, Si4- is bonded in a body-centered cubic geometry to eight equivalent Mg2+ atoms. In the second Si4- site, Si4- is bonded in a body-centered cubic geometry to four equivalent Li1+ and four equivalent Mg2+ atoms. In the third Si4- site, Si4- is bonded in a 8-coordinate geometry to fourteen Li1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Electrochemical Formation of Li-M-(M')-Si Phases Using Multivalent Electrolyte Salt Additives

Lithium-rich silicides (Li 15 Si 4 ), formed during the electrochemical lithiation of silicon, show high reactivity with electrolyte components that contribute to capacity decay, formal lithium loss, and low coulombic efficiency. Recently, the reactivity of lithium silicides was found to be suppressed by substituting a multivalent cation (i.e. Mg, Ca) for lithium that results in the room temperature formation of a ternary Li-M-Si phase. In this study, we explored a range of multivalent electrolyte salt additives (M = Ni, Cu, La, Ce, Sr, Ba, and Ca-Mg mixed salt) in a lithium-ion cell configuration and identified a room temperature electrochemical route to the formation of new ternary and quaternary lithium silicides. Using this method, both nickel and copper salts were found to plate onto the silicon electrode surface upon lithiation. Further, based on refined synchrotron XRD data, multivalent cations with an ionic radius similar to Na (~1.03 Å) or smaller can be inserted electrochemically into a formally cation-deficient Li 15 Si 4 host lattice to form new ternary (or quartenary) phases. The electrochemical synthesis of a new quaternary Li-M-M’-Si phase represents a facile route to preparing and scaling materials isostructural to the Heusler phase and electron-precise Li 14 MgSi 4 phase that results in enhanced cycling and calendar life performance.

25 ENERGY STORAGE↗
DOE OSTI2023

High-pressure behavior of 3.65 Å phase: Insights from Raman spectroscopy

Abstract The 3.65 Å phase [MgSi(OH)6] is a hydrous phase that is predicted to be stable in a simplified MgO-SiO2-H2O (MSH) ternary system at pressures exceeding 9 GPa. Along cold subduction zones, it is likely to transport water, bound in its crystalline lattice, into the Earth’s interior. The 3.65 Å phase consists of Mg and Si octahedral sites attached to the hydroxyl group that forms a hydrogen bond and is predicted to undergo pressure-induced symmetrization of the hydrogen bond. Therefore, in this study, we investigate the high-pressure behavior of the 3.65 Å phase using Raman spectroscopy. We have conducted five distinct compressions up to ~60 GPa using two different pressure-transmitting media—alcohol mixture and neon. At ambient conditions, we identified vibrational modes using complementary first-principles simulations based on density functional perturbation theory. Upon compression, we note that the first derivative of the vibrational modes in the lattice region stiffens, i.e., b1lattice > 0. In contrast, the hydroxyl region softens, i.e., b1OH > 0. This is indicative of the strengthening of hydrogen bonding upon compression. We noticed a significant broadening of vibrational modes related to hydroxyl groups that are indicative of proton disorder. However, within the maximum pressures explored in this study, we did not find evidence for pressure-induced symmetrization of the hydrogen bonds. We used the pressure derivative of the vibrational modes to determine the ratio of the bulk moduli and their pressure derivative. We note that the smaller bulk moduli of hydrous phases compared to the major mantle phases are compensated by significantly larger pressure derivatives of the bulk moduli for the hydrous phases. This leads to a significant reduction in the elasticity contrast between hydrous and major mantle phases. Consequently, the detection of the degree of mantle hydration is likely to be challenging at greater depths.

Geochemistry & Geophysics↗
NASA NTRS2012

Chemical Heterogeneity on Mercury's Surface Revealed by the MESSENGER X-ray Spectrometer

We present the analysis of 205 spatially resolved measurements of the surfacecomposition of Mercury from MESSENGERs X-Ray Spectrometer. The surfacefootprints of these measurements are categorized according to geological terrain. Northernsmooth plains deposits and the plains interior to the Caloris basin differ compositionallyfrom older terrain on Mercury. The older terrain generally has higher MgSi, SSi, andCaSi ratios, and a lower AlSi ratio than the smooth plains. Mercurys surface mineralogyis likely dominated by high-Mg mafic minerals (e.g., enstatite), plagioclase feldspar, andlesser amounts of Ca, Mg, andor Fe sulfides (e.g., oldhamite). The compositionaldifference between the volcanic smooth plains and the older terrain reflects differentabundances of these minerals and points to the crystallization of the smooth plains from amore chemically evolved magma source. High-degree partial melts of enstatite chondritematerial provide a generally good compositional and mineralogical match for much ofthe surface of Mercury. An exception is Fe, for which the low surface abundance onMercury is still higher than that of melts from enstatite chondrites and may indicate anexogenous contribution from meteoroid impacts.

Chemical heterogeneity↗